Let's see...
The essential ones:
Easy Drag To Go - drag an image or link to save them. Very useful for quickly saving images!
Adblock Plus - disables all ads on almost all websites.
Context Search - select some text and you can instantly search for it in any of your search engines (the ones on the upper right)
DownloadHelper - leech media from a page. Saves youTube videos, etc. Very powerful.
Flashblock - blocks all Flash content until you click it. Saves on bandwidth and annoyance!
Grab and Drag - click and drag to scroll a page, with momentum (like on an iPhone.)
Tab Scroller - cycle through the tabs with just your mouse. (right click + wheel)
Tab Mix Plus - added functionality to tabs - locking, progress bars, link clicking...
Delicious Bookmarks - adds your Delicious bookmarks to the sidebar. I use Delicious to access my internet bookmarks from any computer.
The also-useful ones:
Abduction - save page as image (the whole page with the scrolled out area)
Copy as HTML Link - good for webdev. Select a URL and it wraps it in a link tag.
Copy Link Name - copies the text that is a link. (trust me, it's useful sometimes)
Tiny Menu - compacts the "File, Edit, View..." bar into one button.
