World of Goo is a physics/puzzle game & amazingly submersive for a 2D game.

I don't know what is on the Demo but the full version is $19.95 USD. Steam carries it.

Anyway I just noticed that they added a Linux version.

Official World of Goo Corporation Download Centre

2D Boy: Games

I'll pass it on Jephree to the gamers in the linux forums I belong to. Thanks.