Maxit Mag

Here's an interesting idea.  The EZ-Canvas is a clear acrylic panel that snaps right onto your CRT or LCD screen (curved CRT screens probably wouldn't work) and turns it into a touch screen display.  Two sensors at the top of the panel sense the movement of a pen or stylus and pass this information on to your computer to track your actions.

The EZ-Canvas works with screens that are 17 inches or larger, and it'll snap on to any monitor you want to use it with.

See the product page here.