Website usability testing

While looking into some Twitter accounts this morning, I came across a link to Silverback which was posted by quinns. It claims to be an application for Guerilla (or Gorrilla?) usability testing. The application can be used to test software or websites.
The concept is quite simple. On your Mac with builtin iSight, you have your user test your application or website while you record his or hers face. Silverback captures screen and iSight and combines the two in one movie. Movies are sessions stored in projects. The resulting movie doesn't create an eye-tracking heatmap, but it gives you an idea of the users experience. The concept is shown below. Download Silverback here.