This is pretty mindblowing: Graffiti archaeology. Cassidy Curtis, a brilliant artist and programmer, created a little Flash application that lets you view pieces of graffiti around San Francisco -- as they've evolved over time. He got a group of photographers to share their pictures taken of certain areas of graffiti, dating back years. Then he stitched them together into layered images: You can view a particular wall, then peel back each layer like an onion skin, showing the graffiti that existed before it. You can even zoom in if you want to look at something up close.

It's a lovely interface -- neatly evoking the idea of time passing in an urban environment.

(Thanks to Tom for this one!)