One thing I often want to do in a robot is have it be able to find (and maybe follow around) people. But I haven't been able to track down what type of sensor would be good for that. I've seen PIR sensors which detect motion, but they seem to be more for things like alarm systems, where they tell you that "something" is moving in their field of view, but you can't really use them to actually find which direction the motion is coming from.
