ARToolKit is a freely available tool that tracks 'fiducials' like the hiro pattern on the block I'm holding. It reports a 3D position of the fiducial, and I have a laser pointer inside the block of wood with the fiducial on it. By finding the intersection of a line projected from the position of the fiducial and a line from the camera origin to where the laser dot is I can find the range from the camera of the spot where the dot is.
Sort of. It doesn't work that well yet, I have to keep the fiducial in nearly the same place or the depth values will contradict each other. And it is slow, and requires a high resolution camera to work well- a webcam won't be able to track the fiducial unless it takes up a lot more screen space.
The depth map seen at the end of the video is generated from 300 depth points interpolated (crudely).
Music is Ice Cream Diet from Popsicle Theory:
The source code to my app and the still image (imagemagick based) version of the artoolkit camera calibration tool is here:
It's ugly and has a lot of parts specific to my environment, but it mostly works.
Additional writeup and screen shots: