StarCraft 2: Wings of Liberty, for half-off until March 12. You can get the game through Battle.net right now for $20.
Wings of Liberty focuses on the Terran faction (see: future humans) on the single-player campaign, saddled on the back of the series' popular multiplayer component. The second wave of the three-part RTS sequel is due on March 12, StarCraft 2: Heart of the Swarm. A recent multiplayer beta for Heart of the Swarm concluded on March 1, allowing those who pre-ordered and opted in a taste of the new multiplayer changes.