KP Saver
KP Saver is the screensaver that's bound to freak out a friend, or an enemy, if you happen to install it on their Mac. What it does is displays a Kernel Panic as the screensaver. It's quite realistic too. If there is any onscreen video or audio running it cuts it all off making it even more realistic. It even has a fake little Quartz Composer preview so unsuspecting people will test it and be horrified.
PongSaver
PongSaver is a cool little screensaver for your Mac. It is not only cool, but functional. PongSaver is a clock with the left side winning once an hour, and the right winning once a minute.
(download at bottom of linked page)
Fenêtres Volantes
With Fenêtres Volantes, your windows will fly off your desktop. It's a very nice screen saver that will make people look twice when they see your windows flying around.
Soundstream
Soundstream is a screensaver that, at first glance, looks alot like Apple's Flurry but actually reacts the sound via your Mac's built in mic.