Zombie Escape is a Warhammer 40K modification for Imperial Guard versus Zombies
Eric Laurent has written Verdammnis, a “weird World War II” modification for the excellent commercial rules set “Dogs of War.”
The modifications include rules for Nazi scientists, zombies, super soldiers, and werewolves, as well as providing stats for WWII British, Russian, US and German troops.
Dark World War is a set of free wargames rules — actually pay what you want — for playing WWI games that include demonic and mythological creatures.
Assault on Innsmouth is a set of free wargames rules for involving the Lovecraft Mythos. The players are government forces taking on the horrors in Innsmouth. The forces of the mythos are run by the system. The rules are a heavily modified version of the Two Hour Wargames system. There are also expansions at Board Game Geek.
Brimstone is a set of free wargames rules for 15mm to 28mm skirmish battles in Hell. Written by Anreas Udby, the premise is that the Prince of arkness has been slain, and that his lieutenants are battling for control of the underworld. It’s cleverly written, with lines like this:
Panic. There�s also no such thing as morale in Hell. Face it — to have morale, you really need to have such a thing as happiness. In Hell, there are really just shades of panic, hopelessness, and utter despair.
Another part I like is that it turns out that Hell has a caste system and the various denizens won’t always play nice together.
All in all, an imaginative work.