Tree Style Tab:
For tab-freaks like me, it doubles up the number of tabs you can concurrently manage. Shows your tabs on the left side of the browser as a tree (each new link having its referrer as parent). Especially if you have a wide screen monitor, you won't care the horizontal space you lose.
At the beginning brings random web pages when you hit "Stumble!" button. As you go, you rate these pages with "I like" or "No more like this" buttons and it learns the pages you like. You can easily forward pages to friends with a single-line message.
Blocks all the flash content in every page, unless you click on them. Extremely beneficial since flash content sucks up all the cpu you have.
With a right-click, downloads every link on the page you're on, filtered by extension or a regex. It is also nice to download a single link, because it spawns multiple sockets (4 by default, I think) to download the file.
From the times I was actively doing web development:
Edits css in-place, browses/modifies HTML DOM, hides/shows images or their places, and a bunch of very nice features for a web developer.