firebug

so ever since I started developing web applications that utilize Ajax technologies, I've been using Firebug to debug my client-side code and pages. i must say, if you have not used this plugin for Firefox, you have to get it! it's such an awesome tool, even if you're just designing pages. it really shines when it comes to Javascript and the XHR. the tool allows you to view the individual HTTP requests that are made when loading a page as well as the XML HTTP requests that are made during the experience on a page. It also allows you to interactively view the current page's DOM, view/edit the CSS from the page and view the loaded Javascripts. This is a must-have tool for any Ajax developer (in addition to having Firefox).