To open the console press [CTRL]+[SHIFT]+[~] during a game and enter any of the following codes. Press Enter to activate, then [CTRL]+[SHIFT]+[~] again to close the console. Remember that you can use the console during cutscenes and also while you pause the game by pressing the ESC key.

Note that you need the @ symbol and one space in front of every code for them to work, as shown:

Code
@ alliwantforxmasisa
@ aplethoraof
@ bythepowerofgrayskull
@ ihavethepower
@ dontfearthereaper
@ castratetheheathens
@ gimmegimmegimme
@ timeisonmyside
@ yourbulletscannotharmme
@ ragebuilding
 Effect
Summons one creature at no soul cost (see list below)
Summons four creatures at no soul cost
Full health
Full mana
Get 32 souls
Wizards can collect red souls
Gives you a specific spell
Reset spell timer
You are invincible
Unlock LVL 9 creatures and spells

Creature List

Persephone: manahoar, druid, ranger, shrike, scarab, troll, gnome, gremlin, mutant, ent, dragon

James: manahoar, trogg, earthfling, gargoyle, basilisk, taurock, flummox, ikarus, boulderdash, jabberrocky, rhinok

Stratos: manahoar, frostywolf, sylph, brainiac, vortick, squall, storm giant, seraph, flurry, yeti, silverback

Pyro: manahoar, cog, flame, spitfire, tickferno, firefist, pyromaniac, pyrodactyl, bombard, warmonger, phoenix

Charnel: manahoar, scythe, fallen, locust, necryl, blight, netherfiend, deadeye, abomination, styx, hellmouth