Railgun 2000 is a set of free wargames rules for science fiction armored combat. You need the Fistful of TOWs rules to play.
Rules: Science Fiction
Savage Survive Skirmish Rules
Savage Survive Skirmish is a set of free wargames rules for games set in a post apocalyptic winter.
Knight Models’ Marvel Universe Rulebook
Knight Models offers its Marvel Universe Rulebook free for the downloading.
Genestealer Cult and Space Marine Chapters For 1 Page 40K
One Page 40K is a simplified version of the seminal miniatures wargame. The rules now offes data for Space Marine Chapters and Genestealer Cults.