Mine are:
Colorzilla (PhotoShop-style eyedropper to grab colours from browser content)
MeasureIt (a simple ruler - measure browser content in pixels)
WebDeveloper (a whole heap of features, my favourite is the live CSS editing)
Lorem Ipsum Content Generator (nuff said)
Performancing (blog editor as a sidebar)
After posting this I noticed Jeremy Zawodny’s similar list …
What, no firebug?
http://getfirebug.com
:D