The absolutely phenomenal sci-fi epic Starcraft 2: Wings of Liberty is free to play.
As RTS go, this one is legendary in it's vast ESports presence and it's engaging, slightly campy, low budget high production sci-fi story about a Texan beefcake looking to cure his love of an alien parasite. Without a doubt this tragic love story is one of the all time most iconic stories in gaming history and with good reason, fantastic performances from all actors and facial modelling and animation make even minority of gamers who loathe the gameplay slog through it to watch the epic unfold.
I can't recommend this enough, it's completely free so there's nothing to lose. If for some reason you hate love, stories, acting and camp tragedy then at the very least you now own the best RTS of the last 10 years.