Star Wars Silent Death is a set of modifications for the excellent Silent Death rules for the Star Wars universe.
Rules: Science Fiction
Battleshift Space Fleet Combat
Battleshift is a set of space fleet combat rules for big fleet battles.
Lights In The Sky Starship Battles
Lights in The Sky is a set of free wargames rules for space battles.
Trenched Rules
Trenched is a set of free wargames rules for squad level combat. In spite of the name, it is not a WWI rules set. The example photos are of Warzone science fiction figures.
Shock Force Starship Troopers
Shock Force Starship Troopers modifies the Shock Force rules by Demonblade for the Starship Troopers film universe.