The latest version of KeyLemon introduces a Firefox plug-in called LemonFox that lets users log in to social-networking services using their Webcam.

Instead of typing your password, KeyLemon 2.2 associates your face with your profile, and then regularly checks to make sure that the person sitting in front of the computer matches the image attached to that profile. If it doesn't think they match, the computer takes a photo via the Webcam and then automatically goes to hibernate.

I think this is a good tool for adding an extra layer of security to your computer for a reasonable price.