Here's something neat: hybrid tower defense/FPS. Just found this game on Steam's update news, it was a F2P weekend, and for tomorrow it's 75% off, making the full bundle (the game and 2 x DLC (and the soundtrack)) only about $4.50. Here's a review on PGN:

Sanctum Game Review - PGN
note that some content has been added since this review, and there's probably been a few bugfixes too.

Not a ton there, but more than enough to make it worth a $4.50 deal (if you're quick )