The MouseJiggler is a USB device that simulates a jiggly mouse. This means the computer thinks the mouse is live and the screensaver/password prompt won't come up.
(via Josh's del.icio.us)
Update:
Tom sez, "Here's a program I made that does the same thing as the USB key. It checks the cursor position every 5 minutes and jiggles it if it hasn't moved since the last time it checked. Runs without administrative permissions so it'll work everywhere."
Update 2: Derek points out that this was the subject of a recent xkcd strip.
