SupCom with FA Expansion is good as said above
StarCraft
Total Annihilation (Old but epic)
C&C RA2 (Yuri's Revenge if you can find it)
Dawn of War (Warhammer based)
Star Trek Armada (Again if you can find it and you like Trek)
Sins of a Solar Empire (Best space based RTS out there, has long playability, my record game goes onto about 2 days worth of play with 4 players. I've played it with Entrenchment)
StarCraft
Total Annihilation (Old but epic)
C&C RA2 (Yuri's Revenge if you can find it)
Dawn of War (Warhammer based)
Star Trek Armada (Again if you can find it and you like Trek)
Sins of a Solar Empire (Best space based RTS out there, has long playability, my record game goes onto about 2 days worth of play with 4 players. I've played it with Entrenchment)