Event Horizon is a set of free wargames rules for quick play starship combat. It is played on a grid board.
Rules: Science Fiction
Inquisitor Rules 40k Universe
Inquisitor was game produced by Games Workshop set in the Warhammer 40K Universe. Curiously, it used 54mm figures.
The game is out of print, but a rules archive is at the link.
Star Wars Full Thrust Rules
Full Thrust is an excellent set of starship combat rules — my personal favorite, in fact. Here are a set of modifications and statistics for playing Star Wars battles.
MuTaTioR Le choc des MuTaNs (Mutants: Shock of the Titans)
MuTaTioR Le choc des MuTaNs (Mutants: Shock of the Titans — that’s as close as I can get the translation) is a set of free wargames rules in French for combat between mutants in an arena. Google translate does a pretty good job of making sense of this in other languages. One great thing is that it includes rules for creating mutants — shades of Gamma World and to some extent, GW’s Realms of Chaos (I vaguely remember creating mutants for that game).
Big Burn Fleet Actions
Big Burn is a set of free wargames rules for fleet actions in space.