Press ~ ( above TAB ) while playing to display the console window. Enter the following codes:
Result - Code
All weapons will maximum ammunition - loaded
All weapons - allweapons
Breathe underwater for 999999 seconds - amphibious
Disable flight and no clipping modes - walk
Flight mode - fly
Freeze current screen for indicated amount of time - freezeframe [seconds]
Maximum ammunition for all weapons - allammo
No clipping mode - ghost
Return to life when killed - reincarnate
Scale player size; "2" is default - changesize [number]
Set gravity value - setgravity [number]
Set game speed - setspeed [number]
Teleport to targeting reticule - teleport
Toggle God mode - god
Unlimited ammunition clips for all guns - infiniteammo
Level Select
For Level Select, complete the game in Campaign Mode under Easy difficulty.