Userfly

ReadWriteWeb.com posts a review of Userfly, a  "a simple way to test your site's usability for free - with one line of code. The deal is you add the Userfly JavaScript code to your site and all visitor interactions with your site are then recorded for later playback.

A pretty powerful tool for answering the question "What are users doing on my site?", though there still needs more traditional testing to answer the "Why are they doing that?" questions.

userfly.com from Chris Estreich on Vimeo.