With the version 1.0PR of Firefox, two more noteworthy extensions have popped up!
Sits quietly in the status bar, and offers an "eye dropper" style colour picker, with the option to copy the selected colour to the clipboard ready to be pasted into a stylesheet. It also offers a better zoom function. Facilitates re-creating a site look and feel!
Available for Firefox and Thunderbird, adds a simple notepad and an easy way to copy selected text to it. The notepads are just text files. Especially with the option to add the current URL at the end, it makes it very easy to read and "take notes", and be able to find the source again afterwards when processing the text.