Duke Nukem 64

Access the Cheat Menu

At the title screen, press left, left, L, L, right, left, left.

Become Invincible

At the title screen, press R (7 times), and then press left.

Get all Weapons and Items

At the title screen, press R, C-Right, right, L, C-left, left, C-Right, right.

No Monsters

At the title screen, press L, C-Left, left, R, C-right, right, left, left, right.

Get to the Level Select

At the title screen, press L, L, L, C-Right, R, L, L, C-left.