Sanctum 2
Just picked up a bundle pack for 3$ on Steam. I've never heard of this game, nor have I played the sequel, though I am impressed and i've only played 5 minutes. It's basically a first person shooter / tower defense hybrid but it's also run with the Unreal game engine. I have to say, every game I have played by them has run so perfectly and crisp on my computer, and I don't even have high end specs. I mean, they're not terrible either. This game is a must, and you can get a bundle pack for cheap right now. I'm probably going to get addicted to this one very quickly.