Ajax View from Microsoft Research
From the home page:
Goal: The goal of the Ajax View project is to improve developer's visibility into and control over their web applications' behaviors on end-user's desktops.
The Ajax View approach is to insert a server-side proxy (or web server plugin) in-between the web server machines and the end-user's browser. This proxy captures the web application's JavaScript code as it is being sent to a browser and rewrites the code to insert extra instrumentation code. The injected instrumentation code runs with the rest of the web application inside the end-user's browser and can capture performance, call graph, application state and user interaction information, providing visibility directly into the last hop of the user's experience.
Just seen this demo'd at Mix and it looks very interesting.