, it's not the Web Developer Toolbar, although that is very useful by itself. I'm talking about ColorZilla. This is an extention that does quite a bit, but the most important thing that it does to me is that it allows me to point at a part of the page and get it's color (but that is not the reason I like it) and it's class / id identity. Why is this so important, you might ask? In a world of CSS design, this mean that you know what that element is, instead of having to search for it.
complex pages, with complex CSS, porting a site's design is very hard. Take for example my own site, I've a very spesific color scheme that I use, which dictate the use of the following colors: #FDF6E9;#993300; #FFEAAA; #FFEAC0; #FF9933 , etc. When I want to use a web application on my site, I need to convert that to this color scheme. ColorZilla make this porting trivial even in the most complex cases. I'm using it to change FlexWiki's color scheme, and that is a >1000 lines of CSS that I can just jump in and modify thanks to ColorZilla.