It is amazing what you can accomplish with a web cam, cheap laser pointer and a bit of C++ coding. The complete build, along with all the source code, is available on the site. – Link
There are many off the shelf range finding components available including ultrasonic, infrared, and even laser rangefinders. All of these devices work well, but in the field of aerial robotics, weight is a primary concern. It is desirable to get as much functionality out of each component that is added to an airframe. This page describes how a mini laser pointer can be configured along with a single camera to provide mono-machine vision with range information.