Touchscreens are all the rage right now, so it's no surprise that there's a large DIY movement to accompany their popularity. Instructables user Turkey Tek details how to construct your own DIY multitouch display.
Turkey Tek has used an LCD projector and a few inexpensive components (readily available at your local hardware store) to create a 30-inch multitouch display. Here's the basic principle behind the creation:
An acrylic panel is edge lit with infrared leds. When your finger comes in contact with the acrylic, it scatters infrared light out the back where it is visible via infrared camera. As long as nothing is touching the acrylic, very little of the light escapes, instead just reflecting around inside. Image processing takes care of detecting tips of fingers and relaying their location to application software. Since the camera "reads" the whole display in parallel, it is easy to detect multiple fingertips at once, even those belonging to multiple users.
It looks like construction time for this piece is somewhere around the 20-hour mark, barring any major mishaps — though most of it looks to be time spent sanding and soldering. Hit up Instructables for the list of materials and full step-by-step tutorial. When you're done, the device can mostly be used with some cool open-source demos (you can see some in the video above), so it's not the most practical undertaking just yet, but if you're in the mood for some seriously cool tinkering, it's a great project.