I second the Heavy Gear motion presented above, and also throw into the ring
Jovian Chronicles as your Gundam-esque solar-system bound SF setting (and if the giant robot aspect is unwanted its very easy to remove them). The settings in both of these games are quite fantastic.
For a more uber-space-opera game, check out Core Command. The book may not contain quite as much info as one may be expecting, but it is deliberate to allow the GM to put in as much customization as they want. Besides, it's UBER space opera. }
Star Hero is a great toolkit for creating SF campaigns of pretty much any flavour, and is useful even if you don't use the HERO system (though, of course, an expensive option as you won't be using any of the rule bits).
Both above systems, HERO and Silhouette CORE (used in JC and HG and CC), are generic rules that can be adapted to just about any campaign.
Old-school WEG Star Wars can be a lot of fun if you are into SW, and the system works quite well for heroic action, I found.
LUG Trek, keeping on the licenced theme, also worked very well. The ICON system was very usable, and we had a killer GM who made the game a blast }
Still using the ICON system was the Dune RPG, though you will pay through the nose for a copy
Some ones off the top of my head,
Kannik