Getting the Shotgun:
To get the shotgun will require having the keycard for the Security Office which you will be able to find in I.T. Security on the first floor. In I.T. Security grab the keycard on the left and return to the ground of the Lobby and go into the Security Office which is located next to the stairs. There you will find the shotgun sitting on the table along with a book which will have a hint for the safe where you can find the fabrication plan for the shotgun shells.
Getting the Q-Beam Laser Weapon and Ammo (Q-Beam Cells):
Go to the Hardware Labs and into the machine shop. Then go outside and repair the Breach Failsafe (Repair Level 2 required) to the right. Then go back inside and enter the breached section through the Atrium second floor. Use the computer to open the door to the testing range to pick up the Q-Beam Laser Weapon which is capable of firing a concentrated beam of particles that makes the target explode. The get ammo for it you will need the Fabrication plan for the Q-Beam Cells, to get this you must go to the Deep Storage which you will get to naturally halfway through the story. Once inside use the note on the computer on the right to get up the stairway in the next room and up to the data vaults. If you now turn right you will enter the command center where you will find a safe with the Q-Beam Cells Fabrication Plan in the top right corner. The code can be found on a note pinned behind Danielle Sho's computer screen.
Getting the Nullwave Transmitter:
The Nullwave Transmitter is an essential weapon for defeating Nightmares. To get it head for the Atmosphere Control Room when you enter the Life Support through the Cargo bay and look for the maintenance shaft to the back right corner. Now use the Gloo Cannon to get into the Oxygen Flow Control Room. Look at the note to get the storage room code. Return to the room you started in, and use the grav shaft past the electric floor. There is a possibility that a Nightmare is waiting for you downstairs. The storage room is just right from the grav shaft and in a closet you will find the Nullwave Transmitter fabrication plan. The Nullwave Transmitter disables Nightmares and prevents them from launching it's heavy attacks.
Unlock Artemis Golden Pistol:
To get this secret weapon involves unlocking the Golden Gun Side Quest. Locate the corpse of Marietta Kyrkos in Arboretum station and pick up the TranScribe Collectible and Keycard from the body. When you listen to the TranScribe it will trigger the 'Golden Gun' side quest. Now simply follow the quest marker into the Crew Quarters. You can access this area for the first time during main mission 'Gathering Echoes'. The keycard from the corpse will let you enter a cabin with a safe. The code for the safe was obtained from listening to the TranScribe. The safe contains the Artemis Golden Pistol. Picking it up finishes the quest.
Infinite Materials Exploit:
In the early stages of the game you will find recycling machines that can turn unwanted items into materials that can be used for crafting. The following is a duplication exploit that only works on materials. Put the material (best in a stack of 10) that you want to duplicate into the recycle machine and split them up inside the machine. So if you put a stack of 10 in there, split them so each slot is filled with 1 of the material. You will be able to check your duplication gain on the monitor in the middle before you start recycling. Once you start the machine you will be able to collect your duplicated materials on the right.