Cheat Mode:
Hit F2 while playing to bring up the console window. Enter any of the following codes to earn the corresponding cheats:

maxammo: Full ammo for equipped weapon
healme [#]: Restore health to # entered, with 100 being the maximum.
superdeform: Give enemies big heat and feet on tiny bodies. So cute!
flowerpower 1: Like hippie mode for Serious Sam, changes blood to flowers.
playersonly: You are the only one who can move. All others are frozen. Enter again to deactivate.
suicide: Kill yourself.
quit: End game.

Cheat: God Mode:
Edit the user.ini file in your XIII system folder. Change the "Comma=" to "Comma=GOD". Press the Comma during gameplay to toggle God Mode.