Johnny Chung Lee has done a marvellously simple hack which uses the standard Wii remote controller, plus some ballpoint pens modified to emit infra red, to convert any surface into an interactive touch-screen. With multiple pens, you can support multi-touch effects (as seen on the iPhone and iPod Touch), and previously mentioned on this blog back in March 2006.
Here’s an example of his technique in use:
(Make sure you watch the video long enough to see the technique in action; it’s very impressive, especially when combined with a video projector.)
Johnny’s software to make all this work is free, and available here.