My current list of add-ons for my work laptop.
ColorZilla
eye dropper to pick colors from webpage
CustomizeGoogle
disables ads, etc in google search results, direct links in images, other cool stuff
Declicious Bookmarks
social bookmarking
Elastifox
Amazon EC2 helper (don't really use anymore now they have their own interface)
Evernote Web Clipper
Nifty web notebook sort of thing
Firebug
Awesome developer tool, must have for web development
FireGPG
Adds encryption to Gmail
GooglePreview
Adds web previews to google results
IE Tab
self explanatory
PDF Download
mentioned previously
Poster (non-functional in 3.5b)
HTTP testing tool
RestTest(non-functional in 3.5b)
RESTful web services testing tool
Screengrab
webpage screengrabs, similar to Gadwin Printscreen within the browser
User Agent Switcher
helps testing mobile webpages, pretend to be an iPhone
Web Developer
must have for web development
YSlow (non-functional in 3.5b)
firebug extension to help web performance