The Mouse Jiggler has to be one of the strangest USB devices with a real purpose (as opposed to the Sushi USB Drives. It does exactly what the name states - when plugged in it jiggles your mouse around.
Why? Either to make sure your screen saver doesn't come on, or as a practical joke (depending on which mode you select). It was originally marketed towards security professionals and forensics experts who needed to ensure a laptop wouldn't go into a password-protected screen saver mode while they were away.
Link via BoredStop