SurveillanceSaver is a Quartz screensaver for OS X, but not just any screensaver. SurveillanceSaver pulls live streams for unprotected web cams and displays that as your screen saver. It really is quite neat, and contrary to what I believed, it isn't very processor intensive, which means the fan on my MacBook won't annoyingly spin up.

SurveillanceSaver is currently in alpha 0 (I know, scary isn't it?), but it seems pretty darn stable. Go check it out, it's a free download.