I recently had my computer crash, for some unknown reason, so Im now trying out Ubuntu (Linux based OS, Ubuntu Home Page | Ubuntu) But now im using Fire Fox and want to see what add-ons everyone is running.
NoScript
AdBlock
LeechBlock (website blocker. I use it to block an ad company that gets around the Firefox popup blocker by triggering a new browser session to deliver their ads rather than just a popup window)
I use greasemonkey with an exif script for viewing exif data on photos posted to Flickr. Adblock and download helper both kick ass, firefox would be worthless without either of those.