Natural Selection 2
Bought it during the Xmas sale, and it was an excellent purchase.
The game still continues to amaze me, it's one of the few games that actually makes cooperation absolutely necessary on strategic level, while Left 4 Dead did it on tactical level.
Playing in a team that communicates, listens to its commander and actually executes his plans is awesome experience.
The game is not without problems, the HW reqs are rather steep, and the balance is problematic due to asymmetric economies of the alien and marine side.
However, it's a fresh and innovative title the gaming scene needs so much. It actually succeeds where Savage 2 has IMO failed.