To enter these codes, hold Alt and type the desired phrase. Note that any creatures you create in this manner will be given to the computer as well:

FOG? WHAT FOG? - Shows entire game map
BOOST - Maximum Mana
ELMINSTER - All research options
SHADOW - Wraith
BODY GUARD - Guardian
SMOG - Wyrm
YOGI - Ranger
CONCRETE - Stone Golem
ACOLYTE - Basal Golem
FATHER - Cleric
WOLVERINE - Druid
FATAL ATTRACTION - Fury
FLYING MONKEYS - Gargoyle
RAISE DEAD - Ghoul
ALASKA - Gnome
MERV - Griffin
NEEDS FOOD BADLY - Warrior
MERLIN - Wizard
DEAD FLESH - Zombie
MICROSQUISH - Creates Juggernaught

These next three only work on the Harvest of Horrors levels as Redfang The Reaper...

MOTHER IN LAW - Harpy
GREMLIN - Enchanter
JARETH - Goblin

Tip:

Make your creatures while your opponent has a follower on their bloodforge. This works on MOST levels. If you do this it will give you two followers, one at your bloodforge and one at theirs.