My plugin/extension habits are all over the place because I jump between browsers quite a bit. The upside is I'm exposed to plugins/extensions from across the browser ecosystem. These are the add-ons I rely on most:
Firebug (Firefox) -- I can't believe this thing is free. It's hands-down the best HTML/CSS testing tool I've ever used. It's also incredibly handy when I can't remember my own CSS naming conventions.
ClickToFlash (Safari) -- This automatically disables any Flash-based elements. But unlike strict ad blockers, ClickToFlash gives you the option of activating Flash on a piece-by-piece basis. So if you want to watch a movie on a web page but you don't want to see the Flash ads, just click the movie element and that Flash-based part of the page will load.
How about you? Which plugins/extensions do you use?