MouseBot

This is a creation that takes advantage of our old trackball mouse. It uses a motor to spin the trackball either this way ↗ or this way ↙. If you connect it to someone's computer you can annoy the heck out of them by gravitating the cursor toward the top right or bottom left corner!

It's especially good on Macs, as you can see in this video, because in System Preferences you can set the screensaver to start when the cursor goes to a corner of the screen...