Does Team Fortress 2 count since it's now free? (which, coincidently, is why I got it)
If not, and not counting DLC (as I just bought the add-ons to Fallout New Vegas as they were 75% off on Steam), then Avadon: The Black Fortress. I found it by accident a couple of months back and downloaded the free demo. When I played it a bit, I found I really like it, so I bought the full version.
It's a turn-based RPG with top-down isometric style graphics (think Fallout 1 and 2, etc). Quite fun... no bugs that I can find though could still do so much more, gameplay-wise. It is a huge game though.