What open source tools are available for web app tracing/debugging?

I am seeking any open source tools that can be used to trace/monitor a person's web activity in order to determine any code issues for a particular web app?

Basically, I have a web app that I have developed and some 开发者_JAVA百科users are experiencing page stalls and was wondering on what the best way to track down these issues?

---

Enable the debug menu in Safari / Chrome, or install the firebug plug-in with FireFox. I've mostly used Safari; it gives both a timeline of what the page was waiting for, and a profiler for cpu utilization.