This robot, christened Zeitgest, can follow grocery shoppers and hold their items. It uses hoverboard motors to move, a Kinect with OpenCV to sense its environment, and ROS to bring everything together. Also there's an insulated department below to keep items cool. And underglow.
It is pretty hard to sense humans, especially when you have about $50 left to buy cameras and hardware for image processing. We spent a little bit a time trying to use radio waves but couldn't pick up any noise outside the audible spectrum. We ended up using basic color tracking with the Kinect camera.