The developer-related add-ons I use are:

* ColorZilla - lets you get the color of anything on the page by clicking it
* Firebug - does crazy stuff
* IE View - lets you right-click the page you're on, select it, and then that page opens in IE (nice for comparing)
* MeasureIt - lets you measure sizes of different parts of the page, for figuring out margins/paddings/widths/heights
* View Cookies - provides a nice view of all of the cookies for the domain you're on (it puts it in a tab in the Page Info)
* Web Developer - also does crazy stuff

And then the other ones I use that aren't really developer-related:

* Adblock Plus - duh
* BugMeNot - lets you automatically get login info for a site if it exists at bugmenot.com
* DictionarySearch - lets you highlight a word, right-click, and automatically pull up an online dictionary page for that word
* Password Exporter - lets you export/import your saved password list between computers
* Tab Mix Plus - probably my favorite extension ever...provides a lot more tab-related functionality...it's crazy awesome