Monkeyfy, by Michael Schieben, is a public webservice for face-detection based image manipulation. A small bookmarklet allows you to select an image on every website to have it modified by monkeyfy.com. The image manipulation utility detects faces in the images and replaces the them with monkeys. The manipulated images (and a link to the original source) are saved on the twitpic gallery.

http://monkeyfy.com/