Steve Jackson Games has made the rules for the Classic Car Wars game free for the downloading.
I played the heck out of that game back in the day.
Science Fiction
Cardbots :Print-And-Play Card Game
Cardbots is a print-and-play game where you build your robots and then fight with them.
Nighthawks Space War Card Game
Nighthawks is a free print-and-play game of space combat. Remarkably, each player needs just 16 cards.
Collapse Post Apocalypse Print and Play Game
Jordan Goddard Design offers a print-and-play of their game Collapse, a post apocalypse deck building game.
Dr. Who Cosmic Encounter
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.