The Powder Toy

You probably have heard of it. The Powder Toy. The best way to describe it is: nevermind, there isn't. Just download it. Their website server is down (as of 17:06). So download it here:
Windows: http://bit.ly/comzHO
Windows DLL: http://bit.ly/al2pfY
Mac App: http://bit.ly/an34Aw
Sorry no Linux. But it does run great in Wine!