JSMeter isn't a brand-new project, but I just learned of it via a new Microsoft Channel 9 video on the topic. Ben Livshits and Ben Zorn, two of the Microsoft researchers behind the project, made some interesting points during the part of the interview I watched. They explained how they took an implementation of Internet Explorer and changed the source code to see how they could affect the performance of Web applications, like Bing Maps, FaceBook and Gmail.
More from the conclusion of that paper:
Microsoft will be presenting more about JSMeter during the Web Applications '10 conference in Boston in June.