Greg Lipscomb has built a camera flash slave using a photocell and a microcontroller for trigger and timing.
“Anyway, I decided I wanted to make an external slave. I thought about buying a slave peanut trigger. This is a little device that does exactly what I wanted to build. I would merely connect the flash switch to the peanut. I could not find one, so I decided to build my own.”
See also: Photography Hacks