Post hidden gems of the Steam Sale
Quantum Conundrum - $1.79
http://store.steampowered.com/app/200010/Quantum_Conundrum/
It's a cute first person puzzle platformer about dimension-shifting made by one of the creators of Portal and Narbacular Drop. It's got some pretty clever puzzles and level design. It's only about 6 hours long so it can keep you entertained for an afternoon.
I was about to post Marlow Briggs, but incredibly, it's not on sale for a dollar like it is during every other major Steam sale.
It's still only $5 and worth it for that price.