From within the game, press CTRL + SHIFT + ~. This brings up a small text icon in the bottom left of the screen. Then, type ''@ '' (without the quotes, but WITH
the space) and any of the following cheats. Example: @ alliwantforxmasisa firefist

alliwantforxmasisa    - Summons one monster of the given type without using a soul. 
aplethoraof           - Summons four monsters of the given type without using up souls.  
bythepowerofgrayskull - Full heal 
ihavethepower         - Full mana 
dontfearthereaper     - Gives you 32 (I think) souls 
castratetheheathens   - Wizard can collect red spirits 
gimmegimmegimme       - Gives the wizard a spell of the given type. 
timeisonmyside        - Resets the spell timers 
yourbulletscannotharmme - The wizard (and only the wizard) is invincible        
mywingsarelikeashieldofsteel - Same as above 
ragebuilding          - Levels you to level 9 and gives you all the spells and creatures accordingly (only in local multiplayer)
         

Thanks to Revolution reader Asmor!