Machinarium is terrific. I picked it up for $5 earlier in the year when they were having their pirate sale. The entire style of the game is wonderful, from the art and music down to the characters. If you loved the old Sierra Quest puzzle games, you'll enjoy Machinarium.
My only complaint was that it felt rather short. I'd have been upset if I'd paid $20 for it. But at $5, it was a great bargain.