I know there are a ton of these out there and thought this might be a good way for some people to learn about any that are handy or cool that they were heretofore unware of. I'll get things started.
BBcodeXtra - I use this one all the time and find it to be invaluable, and anyone who frequents message boards will almost assuredly agree. In text boxes such as this one, you highlight text that you're typing, right-click on it, and the BBcodeXtra menu lets you apply formatting such as bold, italics, blockquoting, etc in the very common bbCode format. If you're wanting to make a link, just copy the URL and select Make selection URL. It's just insanely handy, and you can also use it to apply HTML and xHTML code. Here's a screenshot of it at work:
ColorZilla - Very handy for getting hex color codes from images in your browser.
BugMeNot - Very hand for bypassing registration on sites that want you to register to view content. Just right-click in the username box on the sign-in page and select the BugMeNot option and it checks the BugMeNot database for valid sign-in info and automatically applies it. Login accounts are all user-submitted, so don't feel like you're hacking into someone's account or something.
Allow Right-Click - Some sites won't let you access a right-click menu to keep you from copying text or images. This does a good job of bypassing that.
Gmail Notifier - I don't really use this one anymore since I installed a Gmail checker plugin for Trillian (fantastic program), but I used to use it all the time. Does what you might think -- provide your login info and it will let you know when you have a new email in your Gmail account. For security purposes, you have to re-enter your info after closing Firefox, though I think you can have that info saved.
I'm sure lots of you have some cool ones I've never heard of, so let's hear 'em.