God Mode
Press "t" to bring up the chat bar, then type "there can be only one".

Invulnerability
Press "t" to bring up the chat bar, then type "never tell me the odds".
Note: that this doesn't work when the enemy is blasting away with his 20mm cannon.

Reload Weapons and Fuel
Press "t" to bring up the chat bar, then type "i'll be back".

Restore Health
To be able to enter these codes you must hit "t" to bring up the chat bar, then type "we can rebuild him".

Win the Current Level Instantly
Press "t" to bring up the chat bar, then type "it's not my fault".