Installation 42 is a free print-and-play deck building game with a science fiction theme. Looks good.
Science Fiction
Pocket Imperium Free Card Game
Pocket Imperium is a free card game of the Expand, Explore, Exploit and Exterminate genre. It’s designed for three players (not many of these) and uses just 18! cards.
Bosconian D Solo Board Game
James Catalano offers Bosconian D, a free solo boardgame based on the classic Namco video game. To find it, you have to scroll down to the bottom of the page, past his (very cool) retro video games downloads.