Cheat Mode:
Press the Keypad Enter button to display the console window. Then, enter one of
the following codes.

Code           Effect  
superman       God mode        
teamsuperman   Team God mode    
shadow         Invisibility   
teamshadow     Team invisibility   
ammo           Unlimited ammunition   
refill         All inventory items   
autowin        Win current mission   
chickenrun     Chicken grenades  
UnlockHeros    Unlock all heros 

And these are for single player.

fillmygarage      All vehicles 
worldtraveller    All maps 
winbycheat        Automatically win the level 
monstertruck      Large wheels
balloonuts        Small wheels 
iceland           Poor grip 
stickyroads       High grip 
moonwalk          Low gravity 
gokartz           Little wheels  
bigfootz          Big Wheels  
bigheadz          Big Heads  
boxerz            Big hands and Feet

Thanks to Revolution readers Ghost Dog and MoverPrime!