Duke Nukem 64

Cheat mode:
Press Left(2), L(2), Right(2), Left(2) at the main menu to access the cheat menu in the NTSC version of the game. For the PAL version of the game, press Left, Down, L(2), Up, Right, Left, Up.

All items:
Enable the "Cheat mode" code. Press R, C-Right , Right, L, C-Left , Left, C-Right, Right at the main menu in the NTSC version of the game. For the PAL version of the game, press R, C-Right, Down, L, C-Up, Left, C-Right, Left.

Invincibility:
Enable the "Cheat mode" code. Press R(7), Left at the main menu in the NTSC version of the game. A sound will confirm correct code entry. The invincibility option may now be enabled on the cheat menu. For the PAL version of the game, press R, C-Right, R, L, R(3), Left.

No monsters:
Enable the "Cheat mode" code. Press L, C-Left, Left, R, C-Right, Right, Left(2), Right in the NTSC version of the game. The sound of a siren will confirm correct code entry. Monsters may now be toggled on the cheat menu. For the PAL version of the game, press L, C-Up, Left, L, C-Down, R, L, R.

Level select:
Enable the "Cheat mode" code. Press L(3), C-Right, Right, Left(2), C-Left in the NTSC version of the game. A level select option will now be available. For the PAL version of the game, press R, L, R, C-Down, Right, Up, Left, C-Up.

Unlimited jet pack fuel:
Enable the "All items" code while flying with the jet pack.

Huge Duke:
Start a game in co-op or deathmatch mode. Enable the "All Items" code, as well as the "Friendly Fire" option. Have one player shoot the other player with the expander weapon. Enable the "Invincibility" code before the first player explodes, then disable "Invincibility".

Tiny Duke:
Start a game in co-op or deathmatch mode. Enable the "All Items" code, as well as the "Friendly Fire" option. Have one player shoot the other player with the shrink weapon. Enable the "Invincibility" code, then disable "Invincibility" to get a small version of Duke. Note: Weapons can not be used in this state.

Energy refill:
Destroy any water source, including fountains, fire hydrants, and toilets. Stand in the water and hold A to slowly restore energy. Alternately, stand in front of a toilet or urinal and press A.

Hint: Fire protection:
Continually jump while over hot spots to avoid damage.

Hint: Entering the L.A.R.D. police station: 
Locate the tunnel in the Freeway level. There is a large box on one side of the tunnel. Jump on top of the box, and from there jump on to the ledge. On the far side of the ledge there is a pair of night vision goggles. Jump a little bit above the goggles to enter the vents of the L.A.R.D. Police station. Use the night vision goggles, and follow the air ducts and shoot through the fan on the right side of the wall; or keep going and break the second fan that Also on the right side of the wall. Duke may now explore the interior of the L.A.R.D. Police Station.

Hint: Hear Butthead's voice:
Begin game play on the Duke Burger level. Locate the drive thru sign on the outside of the building. Approach the "Order Here" sign and press A or move around to the center and back of the sign and press A. Butthead's will say "Go away. We're like closed".