Various Cheats:
Press the ~ key then enter one of these codes

god God Mode
allammo 999 Shots for All Guns
fly Fly Mode
ghost Walk Through Walls
walk Turns Off "fly" and "ghost"
behindview 1 3rd Person view

Pick Level:
Type in ~ then open Level (Where level is a level below)

AR-Asteroid
AR-Barracuda
AR-Luna
AR-Orbital
AR-Sequoia
AR-Sky
AR-Tut
Autoplay
Entry
Nerf
Nerf-Plaza
PM-Amateur
PM-Asteroid
PM-Barracuda
PM-Champion
PM-Luna
PM-Orbital
PM-Sequoia
PM-Sky
PM-Tut
PMX2-Luna
PMX-Asteroid
PMX-Luna
PMX-Orbital
RR-Amateur
RR-Asteroid
RR-Barracuda
RR-Champion
RR-Luna
RR-Orbital
RR-Sequoia
RR-Sky
SH-asteroid
SH-Barracuda
SH-Luna
SH-Orbital
SH-Sequoia
SH-Sky
SH-Tut