WOT:Web of Trust
The greatest thing since sliced bread, OH I mean um, my favorite add-on that helps you stay safe online, by the help of its users, through a voting-like system. You rate websites from very unsafe to very safe in four categories, and earn trust, based on the number of users who agree with you, to ensure accuracy. In addition, blacklists, and whitelists, such as websites certified by Trust-E, or websites in Phishtank, change the reputation. AND, on top of that, algorithms are enforced, that are used to detect people abusing the system. If you are looking for me, I'm wehaveitall on the forums
Stumbleupon
It slows your browser to a crawl, but still it's pretty cool It lets you stumble upon a page liked by another user, so you can always find cool new stuff.
Quick page zoom
The name says it all.
Download Statusbar
An add-on that makes downloads more efficient, by showing them in a nifty bar at the bottom, allowing you to move your cursor over it and find all the details you need to know about it, instead of having an annoying download window come up.
Some of these are FIrefox only
