The classic of Science Fiction RPGs is of course Traveler. I know little about it honestly, but as far as I know, it has a small but loyal cult following since the early 80s and is still around, which is impressive in its own rights.

What I personally can recommend however is Transhuman Space. Not only is the game so... colourful in ts options, it is still somehow relatively realistic and a ngh perfect match of setting and system (it uses GURPS, which delivers great game mechanics as always).