Photography enthusiast Nick Pagazani was hindered by the range and fixed location of his camera's flash. Since his camera has no connector for an external flash, he needed a light-activated slave trigger to fire a remote strobe. It had to ignore the pre-flashes used for red-eye correction and fire only on the main flash. His solution: Use a microcontroller to count pulses from a phototransistor and trigger the strobe at a switch-selectable count.

Case #96: Nick Has an Illuminating Flashback at Design News

I think this is a little too complex for most people (heck, it's way too geeky for me) but it's a neat project to look at.