I've recently written some software which can be used to turn the Minoru webcam into an inexpensive ranging sensor. This could be used on any robot with an onboard PC of one kind or another (such as a netbook), and should perform well even on older hardware provided that it has at least a single USB2 socket.
http://code.google.com/p/sentience/wiki/MinoruWebcam
This uses a sparse feature based matching method which probably wouldn't be good enough for object recognition, but might be useful for navigation and obstacle avoidance.