My favorite underdogs and hidden gems on Steam currently available for small prices:
Fight the Dragon - $2.99 (80% off)
Max: The Curse of Brotherhood - $2.99 (80% off)
Marlow Briggs and the Mask of Death - $2.25 (85% off,
Steam key from GamersGate)
Qbeh-1: The Atlas Cube - $2.49 (75% off)
How to Survive - $2.24 (85% off)
Fight the Dragon is a simple Hack and Slash Action RPG with random loot and leveling, the graphics and gameplay remind me a bit of Zelda (or Oceanhorn), but what makes it special is that it doesn't feature randomly generated dungeons and huge areas, but instead the heart and soul of it are small community created adventure maps with short stories (5-30 minutes), and it comes with a level editor that allows everyone to start building their own levels. Kind of like a coffee-break NWN, Zelda-style.
Max: the Curse of Brotherhood is a nice, kid-friendly puzzle platformer that in some ways reminded me of the 90's game Heart of Darkness.
Marlow Briggs is a God of War clone with an intentionally cheesy B-movie story. It's solid and a lot fun if you don't take it seriously.
Qbeh-1 is like Blocks That Matter in first person view, with relaxing atmosphere. (The only thing that's a bit off about it is that you can die and be reset to the last checkpoint if you miss a jump and fall off the map. But since the maps aren't that big it's not that much of an issue.)
I believe How to Survive was in a $1 bundle recently, so maybe it's not that attractive anymore? I still think it's worth it though.
And if you are less picky, you might also get something out of
The First Templar,
Demonicon - The Dark Eye, and
Blood Knights. They're not really hidden gems and certainly not for everyone, but I've had fun with them despite their flaws (sometimes I'm easy to please). ;)