Portal2: Only played a starting level but I loved Portal 1. And the credit song is great too!
Bought Civ3 complete in the flash sale cause one of my discs is broken, managed to restrain myself on other sales.
Been playing Dungeons of Dredmor a lot the last few weeks and it's great. I can really recommend it, it's a brilliant roguelike. Turn based blis, but difficult. Prepare to die
It's at 60% discount now so they;re basically giving it away at 2 euros or something (in the UK, I get only GBP in steam now)