Instant kill:
To instantly kill someone with a shotgun without having to aim at their head, use TK to levitate them as high as possible without multi levitating. Walk almost, if not directly under them, and fire the shotgun. It should automatically aim at their head and blast it off.

Exploding enemies:
When you kill somebody, take a sniper rifle and aim around his body until you find one of his grenades. Zoom into it, then shoot it. His body will explode.

See dead enemy spirits:
You can actually see the spirit of the enemies that you kill. To do this, you need to have the Aura Vision ability. Then, kill a guard while in Aura Vision mode. You will see their "spirit" float up, cartoon-style.

Special skin abilities:
Some of the character skins have special abilities. For example, there are masters of Psi powers such as Barret, Pyro, and Lenov. They can do many more things than Nick with their powers. If you were to unlock their skins, you will also get those abilities.

Edgar Barret:
Can carry all MPs, has the ability to carry very heavy objects (boxcars, trains, crates, etc.; preferably in the Barret boss fight), Psi drains slower than usual.

Jov Lenov:
Can Mind Control anyone, can make anyone commit suicide at anytime, Psi drains slower than usual.

Marlena Pyro Skin:
Her Pyrokinesis can burn faster, fire goes through most objects or until it hits a wall, Psi drains slower.

The General Skin:
Has all the powers listed above.

Mainframe code:
On the level where you fight Jov Leanov, there is no exact code to get into the mainframe. The code is different in each game. The way to get the code is to use your remote viewing and go into the room all the way at the end of the hallway. Once your remote viewer is in it, you will see people standing at a computer. Look at the computer screen to see the code needed to access the mainframe in big numbers.