Net Epic is a set of free wargames rules based on the old Games Workshop Second Edition Space Marine.
Rules Crusade is a complete rewrite of the Warhammer 40K, Mordheim and Necromunda wargames rules to use the old Milton Bradley Space Crusade system. Its well done, but I’m not sure why Sean Patten went to all the effort. I’m also not sure why GW’s hordes of lawyers haven’t descended upon him and turned him over to the Inquisitors.
Turnsignals on a Land Raider is a very funny, very professionally done cartoon satire of Warhammer 40K. The strip focuses on a newly recruited group of plastic figures from “The Emperor’s Pointy Sticks” chapter of Space Marines.
Rocket Ship Games has a set of tokens and markers for Warhammer 40K free for the downloading.
One Page 40K Rules are a set of simple rules for the 40K Universe. The authors write:
One Page 40k is a single-page ruleset designed to be fast to learn and simple to play. It serves as a great introduction to the Warhammer 40.000 universe for people who are new to the game, and as a way to play quick games for veterans of the franchise.