If you have a Canon compact camera running the Canon Hack Development Kit (CHDK) firmware, you can create a simple shutter release cable using some cheap components. The firmware causes the camera to snap a photograph anytime 5V is sent down a USB cable connected to the camera. You can do this using a USB cable (e.g. the one that comes with your camera), 5V battery, simple push button, and some kind of housing (a metal candy tin, for example). Oh, and you’ll need to be comfortable cutting up and soldering wires. Luo Bo Te over at kisstheasphalt has written up a tutorial on how to put everything together.
DIY Shutter Release Cable For Canon Cameras with CHDK (via Hack a Day)