I paid $1 for La Mulana. I did not expect a super deep 2D metroidvania (not the correct term for this though, this is so much better than any Metroid or Castlevania, it's not funny) that lasted more than 50 hours; crazy puzzles, rewarding exploration, secrets everywhere. A true gem.
I also paid $1 for Volgarr the Viking. Favorite action game ever. I love how every level is divided in puzzley action segments. This is an awesome game for anyone wanting a great challenge. I'm not even a fan of old games like Ninja Gaiden or G&G; I think those rely too much on cheap death and randomness; but this game, every time you die it's your fault, no medusa heads here. Also played for more than 50 hours, even though you can speed run it in 15 minutes.
Finally, I payed $1 for Mark of the Ninja. This is the best stealth game ever. You can approach any fight or challenge the way you want, stealthy, not killing anything, or killing silently, etc. So much freedom, this is great.
This three games are all on sale today, crazy value for the money, it's rare that I have so much fun with a game, so if you haven't played them, go for it