FireBug is by far the best and most complete tool when it comes to Web 2.0 application development. FireBug is a FireFox add-on so it is only a click away when you need it.
Impressions
You get the ability to edit live your HTML, CSS and JavaScript and all changes are reflected in the page as-you-type. This is amazing if you integrate your JavaScript with ASP.NET or other compiled language, you effectively break out of the edit-compile-refresh cycle. Once you are happy with your changes you copy them to your original source file.
I love it because it allows the web developer to experiment with the web site as needed and once the problem is solved he can either copy the fix into the code or email it to another team member.
I first used it with FireFox 2 and had absolutely no problems.
With FireFox 3 just out you will have to install a beta version (1.2.0b3) which brings in some bugs but nothing I can't put up with.
Price
Free.
Conclusion
If you develop or own a web site then this is a must have, 5 stars!
Get Firebug from here: http://getfirebug.com/
