As you might be aware that Internet Explorer Beta 2 was released few weeks back. In this JScript team has added new developer features. We have added some great features to the JScript Debugger apart from improving the existing features. The debugger now has syntax colored sources, support for console.log, break on error etc. You can read more about the debugger features in this post on JScript blog.
Along with debugger, Developer Tools now also has a JScript Profiler to help you make your page more optimal. The profiler shows you report in function view and in call tree view. I specially like call tree view as it can help in figuring out the hot path in the application. Sameer has written a detailed blog on the profiler features.
You should also check out the JScript PM Channel 9 video where our PM team (including me :)) talks about IE8 Beta 2 features.