Yeah i wish they made a new homeworld game, homeworld, homeworld cataclysm, homeworld 2 all rocked.
I was so disappointed that sins of a solar empire was multiplayer only as it was the second 4x game the developer made although I cant find any reference to it. I just remember being disappointed
There's
Homeworlds mentioned above
Nexus Jupiter incident as someone mentioned which is about £5 on steam.
Freelancer also already mentioned
Starlancer which i think was the prequel to this but more of a dog fighting space game.
Freespace 1 and 2, very dated now
Haegmonia, another awesome game. This has an expansion called the solon heritage
Iwar (independence war)
Iwar 2
X series, just google x terran conflict for the newest one
Space force rogue universe wasnt too bad imo
star trek armada 2 was good
sword of the stars
AI war which i just bought off steam but yet to try it, tower defence like game in space
Genesis rising which is a pretty terrible game, but if you need your space rts fix then get it ^^
I know it's not a proper space rts but I thought I would add it.
Metal fatigue - Basically its like C&C but with robots, you destroy the enemy and you can take their better components and stick them on your robots. There are three battlefields in play at once. Below ground,surface of the planet and the sky.
Heres a mod for freelancer
http://www.bsg-galactica.com/ if you can stand the graphics
OMGZ how did I not know about this
http://www.gog.com/en/gamecard/freespace_2/
They released teh source code and updated it!!!
Missed out star wars empire at war, supposed to be a good space rts but it is oldish now. I dont really like star wars games apart from knights of the old republic but from the reviews I had to try it