What are your favorite browser plugins or extensions?

I use firebug all the time. In addition here are two that I like a lot:

pwdhash: automatically encrypts your password with a hash based on the url of the site you are logging in too.

forecastfox: weather forecast at the bottom of the browser

Without a doubt it'd have to be NoScript for Firefox. The fact no other browser has such simple blocking and whitelisting for dynamic content and Javascript ensures I won't be switching browsers anytime soon.

Frankly, given all the thousands of exploits using Javascript as a vector I'm surprised (and somewhat dismayed) it's such an unusual extension.

For Safari, Click to Flash and Saft (Haoli, $), Flip4Mac (Telestream), Silverlight (Microsoft), Flash (Adobe), PDF & Word Browser Plugins (SchubertIT)

Some may ask how I can say Flash is one of my favorites? The criteria I used was how much I use these plugins, and although Flash is problematic, I use it daily for sights like Hulu. Silverlight has brought me streaming Netflix. With out these plugins, the utility of the internet would be much different for me than it is now.

Perhaps one criteria that can be used is how invisible a plugin is. Flip4Mac is like that. I don't even realize I'm using it. It just works. Again it expands the usefulness of the internet for me.

PDF and Word plugins are also in that category of "just works". Although Safari now supports display of PDF files in the browser window, the plugin from SchubertIT displays the first page of a PDF as soon as it is downloaded, unlike Safari which makes one wait until the entire PDF has downloaded. For slow connections or large PDF files you can really see the difference. The Word plugin does the same thing for Microsoft Word documents.

I love the Google Dictionary Lookup & Docs PDF/PowerPoint Viewer extensions in Chrome.

Page Speed, YSlow, FiddlerHook are cool Firefox extensions for web performance analysis & optimization that I find useful.

for Firefox:
adblock: hate ads with a passion...
weave: now called sync. to sync my bookmarks across machines.
noscript: dont want cross scripting badness nor scripting from known bad guys.

others as needed on various machines.