Curiosity is bliss: AJAX Debugging with Greasemonkey
This is a pretty cool site for all those AJAX developers.
It's a Greasmonkey Script which instals a HTTPObject sniffer as a floting layer in your browser.
It's great as you can see all the Posted and Recieved files from the HTTPObject, can also copy and past them into your editor. no more alert(responseText) lines in your code!
To use it you need FireFox and the Greasmonkey Extension.
Look at the home page for a MSIE version and any updates.