Quantum Conundrum (75% Off)

Retailer: Steam

Price: $3.74 (75% Off)

Link: http://bit.ly/KOspku 

If you like Portal, there's a good chance you'll like its creator's latest game, Quantum Conundrum. Mixing the first-person genre with some new dimension-shifting puzzle mechanics, the game is every bit as mentally stimulating as its ancestor but just a little less cruel.