I've been obsessed with The Powder Toy all weekend. It's a simple 2d physics sim where you drop powder of different materials onto the screen, and stuff happens. Seeds, water, gunpowder, and acid all add to the fun. Here's a version you can play in a browser...and a more involved version you can download.
And that version includes cells from the Game of Life, which quickly take over....and that leads me to this music toy based on cellular automata. You place cells in a grid, they move around according to rules, and music happens.
All of this, as usual, thanks to Metafilter.com.