Stephen Lewis's Processing file
<a href=test.txt>test.pde</a>
tracks multiple blobs and turning them into a tone melody.  Clicking
the mouse on any pixel re-assigns that pixel's color to be the tracking
color, but it starts on black, so it can follow shadows on the backlit
table top.  The table is actually a piece of mylar inside a plexiglass
holder suspended above a camera in a garbage can.  All materials
available at Staples, including the garbage can, mylar (or tissue paper)
and plexi.  You can also buy a webcam at Staples.
<p>

His program uses the ESS library which makes audible tones.  ESS is at
<a href=http://www.tree-axis.com/Ess/>
http://www.tree-axis.com/Ess/</a>,
but also listed on the processing help
file under "libraries".  The JMyron vision library is at
<a href=http://webcamxtra.sourceforge.net/>
http://webcamxtra.sourceforge.net/</a>, but also listed directly in
Processing's reference help file.
Expect it to run at about 5 frames per second.
<p>

There's also a brightness tracking library right in Processing's
examples menu, which uses a brute force pixel-examiner algorithm to find
the brightest pixel (no centroid or extents or anything about the
bright-pixel being in a blob, so it's a bit random and jittery).
Expect that to run at around 30 frames per second.