Zero-K is a FREE, multi-platform, open source Real Time Strategy game, which aims to be a dynamic, action-packed, hassle-free experience, full of clever strategies and constantly moving combat. Includes an epic scale, realistic projectile and unit physics, terraforming terrain and hundreds of unique and interesting units.
I'd dare to say that Zero-K is the most immersive strategy game available at this moment. In this game you really are in full control of the battle. The limit's are not the lack of units, a dumb unit AI or not enough control over the battlefield but rather the limits of player's capabilities are his own creativity, reaction speed and strategic inteligence. A truly diverse and great game.
The only con is the rather steep learning curve.