During gameplay press tilde (~) to go to console, then enter the following cheats:

ADDCASH - Get Some Cash
NEXTMISSION - Complete Mission
FAILMISSION - Fail Mission
ROCK - Enable God Mode
GOD - Enable God Mode
GIMMEALLMISSIONS - Enable Cruise Mode
FREECAM - Enable Fly Mode
JUMP - Press Left Shift To Jump
SPEED - Press Left Shift For Turbo Boost
TELEPORT - Press R to Teleport To Random Locations
CAMERAEDITOR - Show Cameras
CARCAMEDIT - Move Camera
SETLIGHT - Show Light Settings
SHOWNODES - Show Nodes
SHOWSOUNDS - Show Sound-Lines
EXIT - Exit Game
CANNON - ?
MINIGUN - ?
ROCKET - ?
AMBIENTB - ?
AMBIENTG - ?
AMBIENTR - ?
CARCAMSETATTACHED - ?
CARCAMSETLERP - ?
CARCAMSETVIEWCAR - ?
CUTEPISODE - ?
CUTMISSION - ?
CUTMISSION - ?
DIFFUSEB - ?
DIFFUSEG - ?
DIFFUSER - ?
FLARESAVE - ?
LENSFLARE - ?
LIGHTEDIT - ?
SAVECARCAM - ?
SETINT - ?
SETSTEPSIZE - ?
SHOWRESPAWNS - ?
SPECULARB - ?
SPECULARG - ?
SPECULARR - ?

Note: This game is either very buggy or beta, for which reason some of the cheats may, or not, crash the game randomly.