I love strategy games, and many of my favourites RTS are free. some of non-stratgey games in this list too.
On Steam
Istrolid - Not traditional RTS, but its one of the best RTS's, I've ever played.
Hawken
Mechwarrior
Warface
Galcon 2
Without Steam
Zero K - (amazing Supreme Commander type RTS, uses Spring engine, which has loads of RTS's).
Cosmoteer - The best shipbuilding game I've ever played
Red Alert online
Open Xcom
FreeCiv
FreeOrion
Battleships Forever