Icarus Miniatures offers a set of preview rules for their Icarus Project science fiction rules.
There are also some starter paper miniatures and unit cards for the downloading.
Rules: Science Fiction
Senator Arjun’s Star Wars Rules
Senator Arjun’s rules are for games in the Star Wars Universe. Each player controls a squad or more of figures. There also are rules for jedi, droids and various villians
Cap’n Arjun’s Starship Rules
Cap’n Arjun’s rules are a generic set of free wargames rules for starship combat.
It looks as though the entire rules set could fit on one piece of paper, so if one page rules are your thing …
Hammer Wars 1.5 One Page Rules
Hammer Wars 1.5 is a set of one page wargames rules that mashes up Warhammer 40K and Warhammer Fantasy: What if a 40K troopship crashed on the Warhammer Fantasy World.
Antares Free Beta Playtest Rules
Warlord Games has released its upcoming Antares science fiction rules for playtesting. Written by Rick Priestly, the pdf looks very good at this point.