There are many ways of controlling a camera’s shutter electronically. Three solutions come to mind: contact closure, USB, infrared and serial.
This is how the camera shutter’s button works. This is just like a light switch; you press the button and the circuit is closed, which takes a picture. This is obviously the most straightforward way to control a camera. The TimeLapseTron uses this.
Some cameras have a dedicated wired remote jack (such as most SLRs). If this is the case with yours, that’s great. Some use standard plugs, for example the Canon Digital Rebel series use a 2.5mm / 3/32" stereo plug, which is cheap and easy to find. If you're feeling creative, you can always connect wires directly to the camera's shutter button. I've got more details in the cameras page.
This is an easy way to control a camera that supports it. There are two inherent limitations, though. You need to be connected to a computer at all times, which can be difficult when you’re on location. You are also limited to the default length of a USB cable (which it typically 6 feet but can be extended up to around 20 feet using active extenders). My Timelapse-O-Matic application uses this.
I’ve never tried this, but cameras that have accessory remote controls will support infrared. A lot of sites and forums will help you find out which frequency to use, and which devices emit these frequencies, but I haven’t tried this. Here is an example. The other problem with infrared is line-of-sight; the device needs an unobstructed view of the camera. If you ever get a chance to make this work, let me know.
Apparently there's a hidden serial protocol hidden in the USB interface of pretty much all Digicams. You can apparently use it to control you camera with a device like a PDA. I've found info about it here, but haven't tried it. If you find an easy solution I'll post it.