You have probably seen face detection at work in programs like iPhoto and Picasa, but what if you could do that performantly in JavaScript? Chinese developer Liu Liu has done the honors, and implemented Face Detection in JavaScript via HTML5 Canvas.
The algorithm is implemented on top of a JS port of a C-based computer vision library by the same author. It works off a grayscale version of the image, and seems to be quite reliable in detecting faces in photographs. The demo then draws a red box around each face in the picture, but the library could be used to do much more interesting things with this data.