I use the Camera Axe (http://www.cameraaxe.com) with great success. Its an open source project that's been around for a while and has a fairly large user and support base.
The device has four phono plugs which can each be configured as an input or output trigger. Simply connecting a wireless device (e.g. PocketWizard) to these ports will let you trigger a camera or just about anything else like a flash.
If you want to save a bit of money and build your own Camera Axe, as I've done, you will need to know how to deal with electronics soldering. A fully assembled version is currently priced at $185 (USD) whereas the build-it-yourself kit is only $85 (USD). Note, the kit does require the additional purchase of an Arduino (http://arduino.cc/en/Main/ArduinoBoardUno) microcontroller board at around $30 (USD).