There are many FireFox add ons out there, so what ones do you use and what do they do?

Here are some of my more useful ones:

Adblock plus - Removes pesky adverts.
Piclens - View your images in stunning 3D!
Fasterfox - Some performance tweaks for FireFox.
IE Tab - Use IE within FireFox for those pesky IE only sites!
ColourZilla - Find out the colour code without having to take a print screen!
MeasureIt - Seen the ideal sig/banner size? Measure It easily