Type in the following codes then press "-" on the numeric keypad to activate

Cheat Mode Enable - bod
Behind view - bucketofchicken
Extra credits - babapapa
Extra life - crystaltips
Instant High Score - dunky
Grenade power-up - albertofrog
Level skip - mrbenn
Unlimited health - zippy
Disable cheat mode - bing
Unknown - bungle
Unknown - triz

Launch the game with these command-line parameters to activate cheat modes:

-mumford - Hodspodkins Mode
-whostayedlate - Rockhard Mode
-whostayedlateagain - EasterEgg Mode

Example:

C:\Expend\go.exe -whostayedlate