Favorite Firefox plugins

I know that nobody asked me to, but I'm going to tell you anyway: my list of favorite Firefox plug-ins. Which is incidentally why I haven't tried out Chrome yet for more than a minute. I can't take a browser serious that doesn't have plug-ins yet. In particular something to synchronize bookmarks. And dictionaries in multiple languages. The other plug-ins that I use are more technical and less critical. Let me give you a more structured view:

• Foxmarks Bookmark synchronizer. I switch computers a lot so I need something like this, and Foxmarks works great. Basic bookmark synchronization hasn't failed me yet, it has a lot of other features that I haven't used yet.
  
• United States English Dictionary and Woordenboek Nederlands(Dutch Dictionary). I use the in-browser spelling checker a lot, with wikis, with GMail and now also with Blogger. Indispensable.
  
• Quirk SearchStatus. A simple way to show basic web page metrics.
• Live HTTP Headers. Can display request and reply HTTP headers.
  
• Web Developer. I needed a way to sign out of basic HTTP authentication (the kind where the web site does not have a log-in page but instead the browser shows a log-in dialog). Normally browsers don't provide the associated log-out functionality. In fact for Firefox this plug-in seems to be the only way. It can do an enormous amount of other stuff that is probably cool also, mainly related to CSS.

What is your favorite Firefox plug-in?