Bring up the chat window by pressing the apostrophe key and type in these codes:

Teamgod-team members are invulnerable
Avatargod-the player you control is invulnerable
StumpyStumpy-changes characters apperearances
Clodhopper-enlarges characters hands and feet
bignoggin-mega big head
5fingerdiscount-replenish ammo
Nobrainer-toggle Al on/off (you cant move through doors,stairs or ladders with this code in effect)
Debugkeys-debug enables or disables keys
Silentbutdeadly-move around with this code on i think its pretty funny
Turnpunchkick-2D character mode
1-900-heavy breathing
Explore-turns the missions victory conditions on/off