I would highly reccomend Jovian Chronicles, from Dream Pod 9. Hard SF-like with some Softer Giant-Mecha bits, it is a solar-system rife with possibilities, neat political units, and nifty mechanical designs. Its ruleset is also well done, with action from character all the way up to capital ship, and a construction system that lets you build ANYTHING (and I mean anything -- none of this 'pick a frame and see what you can stuff into its size).
Also from DP9, and using the same system with different flavour, is Heavy Gear. Set on a single planet (mostly), with an excellent background where neither of the main factions can be seen as good or evil (just different), its a lot of fun, and again full of possibilities.
HERO 5th edition makes for a good ruleset for, well, heroic SF (or other) settings, and Star Hero (the SF sourcebook) is currently being written.
I'd also reccomend LUG Trek if you're interested in the Star Trek end of things (though it's out of print, and I haven't tried the new CODA-based Trek from Decipher). Star Wars 2nd Ed I always enjoyed for the SW-side of things.
Albedo was a lot of fun, and somewhat hard-SF, though the ruleset could've used some work.
If you're looking for totally hard SF, that's a bit different. The silhouette system found in DP9's books would work, but otherwise you might be better off adapting Pheonix Command or something
Oh, I guess I could also mention Space Master -- haven't seen the latest edition, but the older ones were kinda interesting in that they had all vehicle and character combat/resolution rules so you weren't limited to what you did. The system was similar to Rolemaster (well, the same) and had some very fun crit damage tables, but not sure how it would work in practice...
Kannik