I have been playing this so much after I found it at school. It has gone from one person to the next like a virus. It is a downloadable version of the classic falling sand game with many new features. It is currently in a release at v. 45 (not as good as v.46b the new beta version that is availiable.) The game is a complete sandbox of many elements that can be used to create photon lasers, explosions, nuclear reactors, neon signs, and many more things.
The Powder Toy - Download
If you register at thier website, you might find me (same username) on the forums. I would suggest after downloading it to look at the wiki on the website, it has tons of great information about the game and how to use certain elements.