Type in the following codes during gameplay:

• RDALL-Give everything (keys, ammo, health etc)
• RDBETA-Message "Eat me!"
• RDCLIP-Toggle Noclipping Mode ON/OFF
• RDDEBUG-Debug mode ON/OFF
• RDELVIS-Message "Elvis Lives!"
• RDGUNS-Get all guns (the ; and ' keys cycle through the weapons)
• RDHOUNDDOG-Message "Elvis is dead!"
• RINVENTORY-Get all inventory
• RDKEYS-Get all keys
• RDMAXX-Message "Maxx rules"
• RDMONSTERS-Toggle No Monsters ON/OFF
• RDMOONSHINE-XXX Moonshine mode ON/OFF
• RDRAFAEL-Message "For your grandpa!"
• RDRATE-Shows framerate
• RDSHOWMAP-Showmap ON/OFF
• RDSKILL#-Change skill (# = a number between 1 to 4)
• RDUNLOCK-All locks toggled

In multiplayer if you press "y" you will shout "yahoo" and get 1 health.