Digital Media Minute has a popular screencast on debugging JavaScript with Firebug, the popular Firefox plugin. Now there is FirePHP, a debugger for PHP which extends Firebug with an API for PHP web app developers. Six Revisions has a nice overview of the functionality. The jury is out on whether it compares favorably to xdebug and Eclipse PDT, for example.
Still though, as popular as Firebug is with the developer community (I know a lot of non-developers who use it for very simple HTML tasks such as inspecting an element on a page), any tool that extends its basic functionality is a wonderful thing.
