Are you incredibly paranoid? Do you have a Mac with a built in iSight, or other webcam attached?
if you answered yes to both of those questions then Jay Curtis, part of the team behind Your Mac Life, has whipped up Rearview Mirror just for you. It simply pipes the video signal from the webcam installed on your Mac to a small, looping Quicktime movie so you can always see when someone is sneaking up on you.
This little app is more a testament to how easy it is to develop such things on the Mac than it is useful.