Star Hero, using the Hero System is also good- it even has a Star Trek version done up. The system is fairly easy to play, but character creation can be a bear and-- if you're players are new-- will be an entire session by itself. (I'm actually starting up an ALIEN LEGION campaign system using the Hero System-- we're just getting organized and not many seem to know the system, so taking a while-- feel free to check it out. It's listed in the LFGs.) As to the other systems, I'll vouch for both Traveller (done both LBB and d20 versions) and Shadowrun (1st ed thru 3rd, I think), both a lot of fun. Traveller is all space travel and new planets. Shadowrun is Gibson-esque cyberworld, still Earth based tho (unless things changed radically in the time line of the editions). There are also multiple versions of Star Wars out there: the old version (was it Mayfair?), the d20 version, the revised d20 version, and the new Saga edition (I've played all but the Saga-- they're pretty fun). If you're looking post-apocalypse and aren't worried by complexity, Aftermath was awesome. If you want easy and fun-frilled ala D&D early editions, then you can't beat Star Frontiers. Can't think of any others off the top of my head.
