firebug_20070420.jpg
The Firefox Web Developer Extension and Firebug plugins are two very cool tools for editing and debugging HTML and CSS. Jeff Atwood of Coding Horror had this to say:

I can’t imagine debugging a web app without these two essential tools, which transform Firefox into the Visual Studio of browsers. It’d be akin to writing a .NET application in Notepad.

Firebug, pictured above, can even assist with profiling and debugging your AJAX/Javascript code. You can evaluate javascript expressions and see how it effects the document in realtime, as well as view the “generated source,” which comes in really handy when debugging code that populates elements in the page at runtime.

Firefox as an IDE –Link.