Wireshark: Go deep.
Network Protocol Analyzer. Runs in X11. Looks like X11. Geek quotient – hardcore. The point here is you can see traffic. You need that to analyze ajax requests.

YUI Library: Logger
A component of Yahoo’s YUI javascript libraries. Useful if you’re using YUI, otherwise, not so much. I’m using YUI for some things right now but will probably use other tools for debugging anyway.

FirePHP – Firebug Extension for AJAX Development
Use this with Firefox and the Firebug extension. Log messages to the Firebug console from your PHP scripts. It works on the server response side of AJAX rather than the client Javascript side.
The best explanation of what FirePHP is and does was written by the guy who created it. Christoph Dorn – Integrating FirePHP for Ajax Development

There’s also Aptana but I’m taking a break from the whole Eclipse platform.

DHTML eXtensions – professional AJAX components for advanced Web UI – in your PHP, JSP, .NET, ASP and ColdFusion applications

jQuery: The Write Less, Do More, JavaScript Library

jQuery Grid Plugin

Ext JS – Client-side JavaScript Framework

Prototype JavaScript framework: Easy Ajax and DOM manipulation for dynamic web applications

script.aculo.us – web 2.0 javascript – For prototype