To use cheats, pause the game (press ESC) and then just enter them:
All Modes:
painless = God Mode
fogging = Fogging on/off
followme = Move Camera
Sharpshooting Mode:
weapons = All Weapons
ammo = Unlimited Ammo
autoreload = Autoreload on/off
autofire = Autofire on/off
slowmo = Enemies are Slower
slowrocket = Rockets are Slower
Driving Mode:
ghost = Drive Through Walls
nitro = Unlimited Nitro
freeze = Toggle Timelimit
chantastic = Drive Faster
snow = Snow
susonly = Drive Only With Tires
Action/Adventure Mode:
weapons = All Weapons
ammo = Unlimited Ammo
freeze = Freeze Enemies
targeting = Toggle Auto Targeting
laser = Toggle Laser Targeting
ghost = Walk Through Walls
followterrain = Jump as High as You Want
mrbones = Become a Skeleton
shocked = Become Electric
fps First = Person Perspective
fragyuck = More Blood
pillowmode = Small Heads
bighead = Big Heads