Dict
This extension allows me to look up words, abreviations and so on by highlighting them and then selecting "Define" from a right click menu.
Spiderzilla
A brilliant site-ripper.
Disable Target
Stops blank windows opening up when download links are clicked.
Translation Panel
Translates webpages.
Launchy
Launches highlighted links in a number of programs (winamp, IE, Opera, Outlook, Thunderbird, Bit Torrent) via a right click menu.
Magpie
Media grabber.
Somethingawful.com last read extension
So incredibly useful for the forums.
Gmail Notifier
Tells me when i have new mail in my Gmail.
BugMeNot
Access BugMeNot's database from a right click menu, it automatically gets a pass for the current site.
Allow Right Click
Prevents pages that try to stop you using the right click menu from doing so.
Firesomething
Renames firefox to something different each time its opened. Today its Mozilla Supersquid
Sunday8pm
Automatcally filters out anything posted by him on any website ever.
Edit: All these extensions were obtained
here