I can't believe this game hasn't been posted, I really can't believe it, so I am gonna post it.
The Powder Toy is sandbox, Falling Sands-like game, you can play around with materials and create quite cool stuff.
(Description from the Website): The Powder Toy is a desktop version of the classic 'falling sand' physics sandbox game, it simulates air pressure and velocity as well as heat!
Link to the Website:
http://powdertoy.co.uk/The picture doesn't really show anything cool, just a pic I took to show how the game looks like.