Pulse

At first glance this looks like a blog, but it's actually a Flash microsite for the Spanish premiere of the film Pulse.

Once you've hit the Open Video button and finished viewing the Flash movie, the site invites you to "scare a friend" by personalising it with your own text, voice or webcam live recording, and then to send it on to an unsuspecting friend.

I'd say they'd have to be a pretty good friend, or perhaps someone you actually don't like.