I've always liked those pictures that freeze a fleeting instant of time; a breaking glass, a popping balloon. In this project, they use an Arduino to sense a loud sound (the breaking glass or popping balloon) and then triggers a flash to capture the action. It seems to work pretty well. I suppose you could make all sorts of other types of triggers too, like when something breaks a beam of light or something like that. It does require taking the picture in a dark room, so just to go over the top, they control the room lights too.
It looks like a really fun Arduino project. I think we should build it sometime.