The one, I should mention is BugMeNot by Eric Hamiter. Once installed, you can right click on a login box, on several sites which requires you to log on before displaying the content (eg. The New York Times), and choose "Log me in with BugMeNot"! Go ahead & check out BugMeNot.com for this great service!
And I also love DiggThumbs, which will show the thumbnail images of the web site that hosts the digg story. You can see a screenshot at the right side...
[UPDATE - 11/18/2006]
I installed Windows Vista a few days ago and going thru installing all my apps. again. I'm writing down a few I felt I should install immediately to make FireFox 2.0 *usable* for me:
- Install Always Remember Password 0.6: Good for sites like Yahoo, hotmail, who offers "remember password" but does not actually remember it.
- Google Notebook: Indispensable as I take lots of note on the web
- Download Status Bar: A nice small status bar at the bottom for files being downloaded
- Greasemonkey: Holy grail of web customization
- Duplicate Tab: Right click on a tab and create a duplicate of it. I love it!