Firefox Plugins

[Cliphead]

NoScript

[ghl]

Those mentioned + Tamper Data to view/modify HTTP/S headers, etc

[ee61re]

IETab

FlashBlock

AdBlockPlus

Delicious

Weave

[radgid]

for developping/debugging javascript:

firebug

[PRC1964]

Firebug, Page Info and Bork Bork Bork!

[Ardesco]

My List is:

Colorzilla
DownThemAll
Fangs
Fast Dial
Firebug
Firecookie
Firefinder for Firebug
Firequery
Firerainbow
Fireshot
FireXPath
HackBar
IE View
Image Zoom
Pixel Perfect
Safari View Win
Selenium IDE
Tab Mix Plus
Web Developer

Must be the longest list so far

[Sysman]

Shorten URL (Bit.ly)

Simply right click on a page, image or link and the drop down menu shows an option to shorten the URL and copy it to your clipboard.

A nice time saver.

[TheFaQQer]

Download Statusbar
Easy YouTube Video Downloader
YouTube mp3
Colorful tabs
Gmail Manager (if it works with the latest version of Gmail)

[doodab]

springpad

[al_cam]

+1 for adblock
I also use ghostery, I'm a bit paranoid about privacy.