Codelahoma!

Incredibly granular FREE tracing tool for IE6-8 (Firefox coming soon).

From the website:

• Understand performance as real users experience it
• Differentiate between browser or server bottlenecks
• Trace asynchronous JavaScript executions for the full round-trip
• Analyze JavaScript, AJAX remoting, network and rendering performance in real-time
• Save performance data for interactive offline analysis
• Transform Selenium/Watir tests into performance tests and integrate them with your CI environment