Here’s a set of rules for converting the large scale Warhammer Fantasy battles game to a skirmish version.
Games Workshop offers an “essential” edition of its Warhammer 40K rules free for the downloading.
ToyMallet 40 cents is both a set of fairly well-done generic science fiction rules and a brilliant parody of the Warhammer 40K franchise. Written by Jennifer Reitz, it allows you to use any toys you happen to have, instead of spending many hundreds on “official figures.” The photos of the Hello Kittau Neko-Wai Meowarriors are a scream.
Chain of Command: The Horus Heresy is a modification of Two Fat Lardies’ Chain of Command Rules for the Warhammer 40K Universe.
What A Tanker 40K is a modification of Two Fat Lardies’ What A Tanker Game for the Warhammer 40K Universe