N7 armor

Have a saved game file from Mass Effect 3 to get the N7 armor from that game.

Planet Cracker Plasma Cutter

Have a saved game file from Dead Space 2 to get the Planet Cracker Plasma Cutter when you reach the first workbench.

Bonus modes

Successfully complete the game to unlock the following new modes:

Classic: Only classic weapons can be built using blueprints, co-op is disabled, classic aiming is enabled, and difficulty level is Hard. Successfully complete Classic mode to unlock the Devil Horns.
Hardcore: The entire game must be completed without dying. The game can be saved, but a death will reset you to the beginning. Successfully complete Hardcore mode to unlock Retro mode. Note: To prevent the game from resetting if you die, after saving the game, copy your saved game to a USB or alternate memory location to preserve your saved game if you die.
New Game+: You will retain all resources and inventory from your previous completed game. +3 Circuits and MK-II Overclocked Parts can be found throughout the game. It is available on any difficulty level.
Pure Survival: Enemies no longer drop health or ammo, everything must be crafted at the crafting bench, and difficulty level is Hard. Successfully complete Pure Survival mode to unlock a Mega Resources deposit and the MK-II Overclocked Parts Set.

Retro mode

Successfully complete Hardcore mode to unlock Retro mode. Retro mode adds a pixel filter to make the entire game look retro.

Devil Horns

Successfully complete Classic mode to unlock the Devil Horns. The Devil Horns is a special and very powerful weapon, similar to the Foam Finger from Dead Space 2. It has full stats in most categories, and can kill most enemies with one shot. When shooting the weapon, Isaac will say "Bang!" each time you fire it.

Unlockables

Successfully complete the indicated task to unlock the corresponding set(s) and/or suit:

Alien Circuit Set: Collect all Alien artifacts.
Builder's Circuit Set: Collect all blueprints.
Clip Circuit Set and Legends Suit Set: Successfully complete the game on the Casual difficulty.
Clip Circuit Set, Legends Suit Set, and Reload Circuit Set: Successfully complete the game on the Normal difficulty.
Clip Circuit Set, Legends Suit Set, Reload Circuit Set, and Rate Of Fire Circuit Set: Successfully complete the game on the Survivalist difficulty.
Clip Circuit Set, Legends Suit Set, Reload Circuit Set, Rate Of Fire Circuit Set, and Damage Circuit Set: Successfully complete the game on the Zealot difficulty.
Comms Circuit Set: Collect all audio logs.
Crafter's Circuit Set and the S.C.A.F. Deep Dig Suit: Collect all weapon parts.
EarthGov Circuit Set: Collect all EarthGov artifacts.
Research Circuit Set: Collect all text logs.
S.C.A.F. Circuit Set: Collect all S.C.A.F. artifacts.
Silver Adaptor Set and S.C.A.F. Flight Suit: Collect all artifacts.
Silver Attachment Set: Collect all circuits.
Silver Frames Set and Hostile Environment Suit: Successfully complete the demo version of the game with a 100% rating.
Silver Module Set: Collect all logs.
Unitologist Circuit Set: Collect all Unitology artifacts.

Stasis Support attachment

Successfully complete the demo version of the game to unlock the Stasis Support attachment, which can be attached to weapons in the full version of the game. This attachment makes it so when you use a Stasis pack, your co-op partner will also get the benefit of it.

Infinite items

Towards the end of Chapter 8, there will be a shack you are forced to go through that contains a bench, bunk beds, and a power generator. There will be a random item on the ground inside. Grab the item, then leave the shack through the door ahead. Once the door is closed, re-enter the shack, and grab the newly spawned item. It will be either extra money, Stasis or Med pack, or resources. Repeat this as many times as desired. You can sell the items for extra money, or use them to craft new and better weapons.

Any time the game displays "Save and Progress" near an area with item boxes or lockers, collect the items, then pause the game, and select the "Save And Quit" option. Reload the game, and the items will have respawned. Collect the items, select "Save and Quit", and reload the game again. Repeat this as many times as desired. A good place to do this is towards the end of Chapter 5.

At the end of Chapter 13, defeat the Boss. An auto-save icon will appear while you move into a large snowy field scattered with crates. Do not continue to the exit. Instead, collect the tungsten and items from the crates. Then, pause the game, and select the "Save And Quit" option to return to the main menu. Next, select the "Continue From Last Save" option to return to the start of the snowy field area. The items in the crates will have respawned and can be collected again. Repeat this as many times as desired.

All collectible locations

Search the indicated locations to find all artifacts, circuits, logs, and weapon parts in the game:

Weapon parts locations

Search the indicated locations to find the corresponding weapon parts:

Chapter 3

Heavy Standard Frame: Once you are in the airlock and air pressure has stabilized, use kinesis to open the next door. Then, turn left to see the weapon part in front of you near the locked door on the left.
Tesla Core: After letting Norton and his crew inside the ship, the locked door where you collected the first weapon part will be unlocked. Go through that door to where Norton is and turn right. On your way to the objective, near the large doors leading into the rear of the ship is the weapon part.

Chapter 4

Plasma Core: At the beginning of the chapter, turn left to see it on the crate near the bench.
Diffraction Torus: Fly around the circular vessel to get near the humungous structure that is like a half-circle. There are a lot of panels in this area, with yellow outline. Shoot the leftmost one to get the part.
Military Engine: Before using the elevator, check the door to your left. The weapon part will be near the locked door, which requires a tungsten torque bar.
Electric Charge: During the "Explore Greely" optional side mission, in the room with the generator you turn off, the weapon part at the end of the room, on the right.
Telemetry Spike: During the "Explore Greely" optional side mission, enter the room to the left of Dr. Engstom's quarters. Get the key from the reactor room, and use it to open the small door by the crafting bench. Get the tungsten in this room, and use it to make tungsten torque bars at the bench. Open the room to the left of Dr. Engstom's quarters using the tungsten torque bar to find the weapon part inside.
Compact Standard Frame: After decrypting the message during the "Explore Greely" optional side mission, go outside of the ship. Go to the L-shape ship with an eagle logo on it. Use kinesis to open the door. The weapon part is inside.
Precision Tip: After fast traveling to CSM Brusilov, you can see huge wreckage with a number 9 or 6 on it. Near the number are two breakable panels. Shoot both of them, and one of them will contain the weapon part.

Chapter 5

Heavy Standard Frame: Once Skiff docks at the platform, exit and walk along the pathway to the air lock. Use kinesis to open the air lock hatch, and enter. Move in the opposite direction to open another hatch to pressurize the room so that you can board the Terra Nova. Then, grab the weapon part on the right side of the air lock.
Hydraulic Engine: After entering the cargo hull while locating the tram station, go right, and follow the path to a dead end to find the weapon part.
Ejection Field: In the area where you need to activate the control panel to clear the tracks for the tram, the weapon part will be sitting on top of the central control panel.
Conic Dispersal: During the "Conning Tower" optional side mission, you will find all the mid station doors locked. Go up to a control panel on the left side, and complete the electrical engineering interface to connect the power to the doors to unlock one of the doors. Then, enter the room with a workbench. Go up some steps to the left to a control room to find the weapon part.
Rotator Cuff Module: During the "Conning Tower" optional side mission, you will have to climb down a ladder to a lower corridor where a workbench is located. Craft a tungsten torque bar to go through the doorway and climb a ladder back up to a higher level. You will encounter several Necromorphs. Once they are defeated, shoot at a control box to the right of the electrical trap to shut it down. Keep following the locator beam around to another ladder. Before going down the ladder, use a tungsten torque bar to open the room near the ladder. The weapon part and a circuit are inside.
Medic Support: During the "Conning Tower" optional side mission, when you are trying to find a key to the supply room after getting back to the Conning Tower, take the cargo lift up to the supply room, and use the key to open the door. Open the storage container inside to find two circuits and the weapon part.

Chapter 6

Stasis Support: In the hangar where you find the Crozier ship, you can find the weapon part on the other side of the hangar. You can either use kinesis to pull it, or use the shuttle bay to get to the other side; it is toward the rear of the shuttle.

Chapter 8

Survey Charge: When you are looking for the survivors at the beginning of the chapter, look for the heat sources to find the weapon part a few steps from the crash site.
Compressor: After the pipe breaks, keep going until you reach the ground level. Then, turn around and backtrack to go behind the pipe to another red structure, where you can warm up. Open the door with kinesis, and start up the generator to heat yourself. The weapon part is on the bed in this shack.
Stasis Coating: When you reach the end of the chapter, you will see a large circular structure. Do not go inside, as that will start the next chapter. Instead, go around the structure until you find a narrow mountain pass. Go through that pass until you reach the area with the weapon part. It is close to the video console where you get Ellie's message that she will be leaving flares for you to follow and that she is heading to the shelter along with other survivors.

Chapter 9

Canister Recovery Module: The weapon part is in the room with the generator that you need to activate to turn on the elevator after meeting with your partner.
Compact Standard Frame: The weapon part is in the kitchen where an enemy runs away from you. It is on the table to your right as you enter the kitchen.
Explosion Amplifier: The weapon part is in the room where you have to solve some puzzles. From the catwalk at the machinery, look to the left side of the room to see it.
Tesla Core: From the way station, follow the locator beam to a security checkpoint to find the weapon part there.
Heavy Standard Frame: After the battle with the large Necromorph, go inside the outpost. The weapon part is in the second room from where you enter.
Compact Repeater: During the "Supply Depot" optional side mission, at the point where you need to acquire a battery to open the doors where the locator points, insert the battery to the left of the door at the top of the steps to power up the room and gain access to a storage room containing the weapon part.
Scope: During the "Supply Depot" optional side mission, after calling the storage elevator and defeating a lot of Necromorphs, open the storage crate to find the weapon part.
Electrocution Module: During the "Supply Depot" optional side mission, use the elevator next to the suit kiosk before exiting the supply depot to be taken to a tram. In the same area is a bench and kiosk. The weapon part is wedged in-between them, on top of the dispenser.

Chapter 10

Pneumatic Torch: As soon as the cutscene finishes where you find your team, turn left, and go into the room with the audio log in it. In this room is a door that requires a tungsten torque bar to unlock it. The weapon part is inside.
Directed Suspension Field: In a supply depot that has a weapons bench and suit kiosk, the weapon part is right next to the kiosk.
Hammond's Heavy Frame: Enter the excavation site, and go down the ladder after killing the Necromorph that drops on your head. Behind the ladder is the weapon part; use kinesis to get it.
Telemetry Spike: Once you reach the generator to activate it, before turning it on, look on the table near the engineering bench to find the weapon part.

Chapter 11

Repeater: Inside the barracks is a door that requires a tungsten torque bar to unlock it. The weapon part is inside.
Stasis Amplifier: During the "Armory" optional side mission, after exiting the cylindrical room and passing the columns, look on the shelves to your left to find the weapon part.
Plasma Core: During the "Armory" optional side mission, use the elevator inside the room with soldiers. Look down from the catwalk to see the weapon part below. After killing the enemies, use the cargo lift to reach it.
Acid Bath: During the "Armory" optional side mission, follow your locator to the area where the tram is to find a large cache with the weapon part inside.
Stasis Support: The weapon part is in the same cache as the Acid Bath weapon part.

Chapter 13

Compact Standard Frame: At the entrance to the cave are two tents. Turn left from there, and move a few steps forward to find a third tent. The weapon part is next to the third tent.
Weller's Compact Frame: After collecting the Compact Standard Frame weapon part, you will be attacked by Necromorphs. Kill them, then walk forward until you reach the end of the path. Look for a yellow box; the weapon part is behind it.
Ammo Support: When you enter the second cave, there are climbing devices on the far side of the cave. On the barrel by the climbing devices is the weapon part.
Rip Core: Once you reach an area with a broken elevator and a bench to the right of it, look for the weapon part on the right of the bench.

Chapter 16

S.C.A.F. Frame: Near the start of the chapter, continue down the steps to reach the first door after riding an elevator to find the weapon part.
Rail Accelerator: While climbing down a cliff, look on the right side for a piece of corrugated metal debris forming a platform. Use kinesis to grab the weapon part on your way down.
Safety Guard: When you enter the Hydraulics room, look left on the shelves to find the weapon part.
Heavy Standard Frame: After the third ice climbing segment, you will reach a section of cave with several shelves. Look around to find the weapon part; it is just left of the blueprint.

Chapter 17

Compact Standard Frame: In the room with the large glowing circle, the weapon part is on a desk past, and right, of the bench and kiosk.
Stasis Coating: During the "Artifact Storage" optional side mission, the weapon part is on the bridge three levels down from the top floor of the Zero-G environment area. Look for the small alcove jutting out from the main cylinder.

Chapter 18

Acid Bath: At the start of the chapter, look right of the locator path for a cache of weapon parts sitting on a pile of boxes.
Electrocution Module: The weapon part is to the right of the Acid Path weapon part.

Weapons crafting blueprints

To create custom weapons, you must use a work bench. However, you first must have all the required parts to create the weapon. If you use custom parts found throughout the game, you can create completely original weapons with unique stats. You need four main items to craft custom weapons: Frames, Tools, Tips, and Attachments:

source:http://www.cheatcc.com/xbox360/deadspace3cheatscodes.html#.UVv-8aIvmqc