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.
alliwantforxmasisa <monster name> - Summons one monster of the given type without using a soul. Ex - @ alliwantforxmasisa firefist
aplethoraof <monster name> - Summons four monsters of the given type without using up souls. Ex - @ aplethoraof firefist
bythepowerofgrayskull - Full heal
ihavethepower - Full mana
dontfearthereaper - Gives you 32 (I think) souls
castratetheheathens - Wizard can collect red spirits
gimmegimmegimme <spell name> - Gives the wizard a spell of the given type. Ex - @ gimmegimmegimme erupt
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)