GURPS 4e Space + Core Books + Spaceships and you can "roll-your-own" game. GURPS is a detailed, leaning on realistic generic game tool-kit. It takes a little work to build YOUR setting, but the game has a plethora of options that you can tune the mechanics to your liking. Also
Transhuman Space is a great hard sci-fi setting and they have a couple PDFs that update the 3e rules to 4e (Changing Times).
Precis Intermedia is re-releasing the Masterbook series of games, one of which is
Shatterzone. The scans are GORGEOUS!! Easy to read. The setting is basically a bunch of stuff WEG made for Star Wars (extended universe) but was not approved. They polished it up and made a new setting. It's dark, gritty and has piles of stuff. The Masterbook system shows its age a little, but has some interesting mechanics. Worth checking out.
Mongoose is publishing the current version of
Traveller, which is a solid game. It's hard to go wrong with Traveller and the various fans sites and support across the Internets is superb (there's also ALOT). I LOVE Traveller maps! Also, SJGames made a bunch of licensed stuff for Traveller for 3e, and the books have piles of good info for any Traveller game. The GURPS 4e Traveller book is set "in the past" and has a bunch of great maps and setting info.