BlueScreen Screen Saver scares nerds
BlueScreen a screensaver which simulates Windows' infamous Blue Screen of Death, even using info taken from your system configuration to make it as realistic as possible, including the NT build number, processor revision, loaded drivers and addresses, disk drive characteristics, and memory size. The screen saver cycles between different Blue Screens and simulated boots every 15 seconds or so, so load it onto a friend's computer while they're gone and see them freak out when they come back. Or do what I did and just smash their computer with a bat. The effect is the same, but with more broken pieces.