Type them in the " ` " (or "Ö" for Swedish users) console:

Code                           Result
thetrooper                     God Mode
sunandsteel                  Reload Ammo
diewithyourbootson        Unlimited Ammo
stilllife                           Invisibility
revelations                     8 Artillery Shots


Try typing in the phrase "you've got problems", no spaces or apostrophe, and switch to 3rd person view to see what happens. Does not work with all versions.

Command-Line Cheats
Start the game with one of the following command line parameters to activate the corresponding cheat function.

Code                            Result
   /?                              Command Line Help
   /s                              Enable Save Scores Mode
   /p                              [filename] Playback Demo
   /r                               Record Demo
   /d                              Read Resources from Directory
   /sys                           System Information