Here is the Pi being used to power a wheeled-robot that looks for green objects nearby and travels towards them.
The video from the attached web-cam is fed to the Pi, which processes it to look for any green paraphernalia.
Once the system spots a green object, it sends a command to the robot to manoeuvre itself close to that item. If it can't see any green objects nearby, it will turn around until it can.
Its creator says the total cost of materials for the project is about £60.