Duke Nukem 64 

Cheat menu 
At the main menu:
NTSC version (U): press Left(2), L(2), Right(2), Left(2)
PAL  version (E): press Left, Down, L(2), Up, Right,Left, Up.

All items 
Enable the "Cheat menu" code Then at the main menu:
NTSC version (U): press R, C-Right , Right, L, C-Left , Left, C-Right.
PAL  version (E): press R, C-Right, Down, L, C-Up, Left, C-Right, Left.

Invincibility 
Enable the "Cheat menu" code Then at the main menu:
NTSC version (U): press R(7)
PAL  version (E): press R, C-Right, R, L, R(3), Left.

If you entered the code correctly, you will hear a sound.
Now an option for invincibility will appear on the cheat menu.

Level select 
Enable the "Cheat menu" code Then:
NTSC version (U): press L(3), C-Right, Right, Left(2), C-Left.
PAL  version (E): press R, L, R, C-Down, Right, Up, Left, C-Up.

No monsters 
Enable the "Cheat menu" code Then:
NTSC version (U): press L, C-Left, Left, R, C-Right, Right,Left(2), Right. 
PAL  version (E): press L, C-Up, Left, L, C-Down, R, L, R.

If you entered the code correctly, you will hear the sound of a siren. Now an option to toggle monsters 
on/off will appear on the cheat menu.

Infinite jet pack fuel 
While flying with the jet pack:
enable the "All items" code.

Big Duke 
Start a game in co-op or deathmatch mode. Enable the "All items" code and the "Friendly Fire" option:
Now have one player shoot the other player with the expander gun. Enable the "Invincibility" code
before the first player explodes,then disable "Invincibility".

Small Duke 
Start a game in co-op or deathmatch mode. Enable the "All items" code and the "Friendly Fire" option.
Now have one player shoot the other player with the shrinker gun.
Enable the "Invincibility" code, then disable "Invincibility".

Refill health 
Destroy a fire hydrant, or another water source.
Then, stand in the water and hold A to slowly restore health.
Alternatively, stand in front of a toilet or urinal and press A.