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

While in the game, you may enter these commands followed by the "-" key on the numeric keypad:

 

Code	Result
bod	Enable Cheat Mode
mrbenn	Skip Level
zippy	God Mode
bucketofchicken	Behind View
babapapa	Extra Credits
crystaltips	Extra Life
dunky	Instant High Score
albertofrog	Grenade Power-Up
bing	Disable Cheat Mode