Cardbots is a print-and-play game where you build your robots and then fight with them.
Nighthawks is a free print-and-play game of space combat. Remarkably, each player needs just 16 cards.
Jordan Goddard Design offers a print-and-play of their game Collapse, a post apocalypse deck building game.
Gamers of a certain age probably will have fond memories of Cosmic Encounter, which was first released in 1977. The game has gone through numerous incarnations, currently residing at Fantasy Flight. At the core of the game is the interplay between the various interstellar races. The owner of the Dice Monkey site has created some very nice Dr. Who themed alien races for Cosmic Encounter.
Farscape is one of my favorite science fiction series, though like many, its life was too short. You can relive the adventures with the print-and-play Farscape card game.