Steam ain't exactly prime real estate for strategy games.
You'd be better off going to Gamer's Gate - they get a lot of the esoteric European RTS's.
http://www.gamersgate.com/strategy/real-time-strategy
From Steam, though... let's see.
Cossacks: Back to War might be good - it's an AoE2-esque game whose standout features are massive ships that can shoot across the map and armies with literally thousands of units.
Stronghold collection might be good...
AI War: Fleet Command is definitely original, and has gotten good reviews across the board (plus the developer's hurting a bit right now, so...)
Majesty Gold, if you're willing to go back in time a bit, is very fun.
And that's about it for that list. Again, I'd try Gamer's Gate first. Have fun.