Starcraft II goes free-to-play seven years after launch
Highly recommended.  Probably my favorite part is the asymmetrical approaches of the races.  Interested in firepower, maneuver warfare, and positioning?  You'll enjoy the Terrans.  Want to achieve the dream of bloodless (for you, anyway) war?  Try the Protoss.  Want to be an efficient Darwinian nightmare?  Pick up the Zerg.<p>It's this mix of styles, to the point that the races feel like they're in different <i>genres</i>, that makes SCII such a varied game.  And that doesn't even account for different playstyles <i>within</i> the races.