Ok,
Traveller d20 is a solid one. Not only is the book greatly worth the effort, it's price is high but the page-count is worth it, but the on-line support is very present. It's community is active at
http://www.travellerrpg.com/ both in it's message boards and PDF releases. It's a hard science sci-fi game with only soft science thrown in as needed.
In a non-d20 vein I would recommend Blue Planet, from Fantasy Flight Games, as it's one of the best, most interesting sci-fi games I've seen in awhile. Basically most of the action takes place on an Earth colony on the planet Posidonis, which is on the opposing side of a wormhole that was found in our solar system. Regular humans to genetically modified are your choices for race.
Also, in the non-d20 vein, is Transhuman Space, an RPG powered by GURPS. If you want a gritty, realistic hard science RPG then this is the one for you. This too has the modified humans as the races, but the suppliments are, as is SJ Games standard, pretty in depth and enjoyable.
I'll put my recomendation for Spycraft as well, especially with the Shadowforce Archer campaign used to it's fullest. The rules are d20, but the differences are stellar. Personally, especially in the espionage/movie action vein, I recommend it over d20 Modern. Basically if you want James Bond you do Spycraft, but if you want X-files you do d20 Modern.