Ajax / HTTP Debugging
http://www.fiddlertool.com/fiddler/
I forgot to mention earlier that there is an Ajax / Http Debugging tool called Fiddler. It looks at the http sent between the client and server. Could be very useful in debugging web services, xml requests, and thus Ajax.
Fiddler is a HTTP Debugging Proxy which logs all HTTP traffic between your computer and the Internet. Fiddler allows you to inspect all HTTP Traffic, set breakpoints, and "fiddle" with incoming or outgoing data. Fiddler is designed to be much simpler than using NetMon or Achilles, and includes a simple but powerful JScript.NET event-based scripting subsystem.