Thanks to this post at RPG.Net I became aware of Star Without Number, written by Kevin Crawford of Sine Nomine Publishing, a science fiction RPG built along the same rules of the 0E edition of the Worlds-Most-Famous-Fantasy-RPG. Stars Without Number is a complete game, all packed in a very neat and organized PDF. And did I say that it is free? I can only hope that SWN hit the same success of other recent grognard-style games.
P.S.: I couldn't end this post without mentioning Terminal Space, another excellent source for science fiction material. Here we have actually a supplement for 0E Edition campaigns, not a stand-alone game.