Star Frontiers is a pretty good game, and you can still find copies on eBay and at various second-hand places. While most of the material was recycled into D20 Future, I find the original rules to be more suitable for that type of game.
You can get the .pdf's for free over at Star Frontiersman (under 'Digitally Remastered'). They also have ongoing support in the form of a quarterly webzine (up to issue 12) that's looking to go bi-monthly.
I actually don't know that much about the Star Frontiers (besides that its 'Old School') so I'll leave my commentary on it for others more familiar with the works.
I was going to mention Lightspeed, but their website has mysteriously vanished and its apparently been bought up by someone else. They used to have a free sample game download called Instant Lightspeed. It uses the Fuzion engine.
It kinda has a quadrant of the galaxy for everything. The Interstellar Federation, evil Pan Solar Empire, Old Earth Empires and the Frontier. Game Geeks gives a pretty good review [ame="http://www.youtube.com/watch?v=ayFpXu82a2k"]here[/ame].
Like I said though I'm really not sure of the state of the support, and I havn't ran the mechanics. The piece of the setting that I have read seems pretty sweet though. Gives a good framework for almost any kind of Sci-fi game, particularly those with FTL travel.