Freespace (the story in 2 was so damn epic and left a lot of unanswered questions)
Mechwarrior (One of the "best" Sci Fi universes to work with and the whole style is so much better than the Japanese Mecha style)
MechCommander (see above)
Homeworld (Relic's work on the 3D RTS game has never been matched, nevermind bested. Shame there weren't more RTS games based in 3D Space)
Company of Heroes (Brought in some revolutionary concepts and was a solid game in itself)
Nexus: The Jupiter Incident (Homeworld'y type game based more around tactics and capital ships with RPG elements. No sequel ever came out and I think the studio collapsed)
Hellgate: London (Controversial I know but it didn't need to much work to change it from buggy grind to superstar product. Best DirectX 10 implementation I have ever seen. Completely transformed the game graphically)
Aquanox (Unique and different)
King's Bounty (HoMM eat your heart out. This series spanks you

)
Space Pirates And Zombies (Indie game, superb. Cannot wait for their next updates)
Star Wars: Republic Commando (The first was too short even on Hardest but it was pretty simple and epic. Completed it at least 5 times)
Star Wolves (Russians make Homeworld with a non-linear story mode including RPG elements. Epic gameplay if you could ignore bugs... some of which would be nerd-rage infuriating)
Dawn of War (Wondering whether they'll ruin the series by going back to DoW style Space Marines or they'll improve upon the base which DoW2 built)
No, they were expansions back in the good old days. But since each of the SC2 campaigns is going to be huge they decided to release them as independent games. Meaning you do not need to own the current SC2 Terran to buy, install, and play the SC2 Zerg.
Of course, that means you will end up paying more in the long run too.
Then that makes them standalone expansion packs? I'd say so.