During the game, press Enter to bring up the chat bar, type a code, then press Enter again.
1-900: Heavy breathing mode! 5fingerdiscount: Replenishes ammo. avatargod: God mode bignoggin: Big heads on. clodhopper: Big hands and feet on. debugkeys: debug keys fastactionresponseteam : Player's have deadly gasnoises explore: Turns off victory conditions. meganoggin: Huge heads. nobrainer: Disables AI (even your teammates.) silentbutdeadly: Player's have deadly gasnoises stumpy: Stumpy mode. teamgod: Team god-mode turnpunchkick: Enables side scroller mode.