some people are really into the mouse gesture extensions, but I think the best one is the AdBlock. It requires that you tell it which ads or more precisely ad servers, you want to block.
After a few hours of surfing your favorite sites, you have effectively removed all ads from the web pages and they don't come back. Currently Flash ads cannot be removed, or at least I haven't figured a way to block them.
I really like ForecastFox. it puts a little weather forcast in your toolbar or wherever you want it (mine's in my status bar at the botom of the page), and it gets all its info from weather.com, so no spyware crap from things like Weatherbug.
Here's a nice spellchecker. Works in any text entry field on a website.
This is a favorite of mine: Super Drag-n-Go. Once installed, you highlite a word or phrase on a page, then drag it just a hair in any direction, and it automatically does a Google search of what you highlited.