The Hack a Day website has information on how to expand your camera with the CHDK firmware hack. Unfortunately the hack works only on Canon cameras. But the features that it adds (scripting, advanced features like higher shutter speeds, raw formats, etc.) make me wish that I had gone for a Canon camera instead of the Nikon.
The following video (shown on the Hack-A-Day site), was made using a script to take pictures once every 15 seconds.
If you have a Canon camera - definitely check out the CHDK camera firmware hack. The best feature of the hack is that it is completely reversible and so you don't have to worry about bricking your camera.
The Hack-A-Day site and LifeHacker have very good posts on using the hack. The Chdk site is at http://chdk.wikia.com/wiki/Main_Page and Canon camera models supported can be found at http://chdk.wikia.com/wiki/FAQ#Q._What_camera_models_are_supported_by_the_CHDK_program.3F.