Amazingly fun commercial game for Linux!
It's World of Goo. It's like a mad cross between Fantastic Contraption and Lemmings.
The best part (in my world anyway :-) is that the game is available for Linux. I wish more game companies would start doing this. It isn't that hard, and there is a market for it. Nearly as much of a market as for Mac games.
The game has gotten some fantastic reviews. It is light-hearted and bizarre, and the physics simulation based puzzles are highly entertaining. Other comparisons that come to mind are the work of Tim Burton and the video game Worms, more for artistic style than anything about how gameplay actually works.