3E Presents: A Top 21 Things on the Internet: #11 Falling Sand Game
Here, although there are a whole bunch of different kinds. There's also a forum. A very zen entry, I think, that is my favorite example of a pointless timewaster -- at least, pointless besides the enjoyment of it (and isn't that why we're all here?). It's very relaxing but also very addictive.
I like to create a never-ending cycle of fire/plant. But others may have a different approach.