Today Microsoft demoed their AjaxView application in one of the Mix07 sessions. It acts as a proxy and monitors requests, as well as execution time for each javascript function that gets executed. This is really neat for tracking what methods are putting the most load on the client browser. Now all we need is some built-in tracking of memory leaks ;-)
A tech preview will be available within the next 1-2 months.
Update: You can now see the full session where this was demoed online here.