Ask HN: Really detailed session analytics?

The simplest way to do usability testing is to sit next to someone and watch them use your website, but it also occurred to me that Javascript has access to all sorts of salient data: how far a user scrolls down on a page, how long it takes to fill out a form, forms that go unfinished, cursor position, etc.

Is anyone out there trying to collect and visualize this kind of data?