Yet another reason to love Firefox: ColorZilla. The extension adds an eyedropper tool, that will tell you the color of any pixel in the browser, plus a color picker, like the one found in Photoshop. Both of these tools can prove immensely useful to Web developers and designers, as it can save the step of opening a different program when you need to find out the values of a specific color.
Update: I noticed an odd problem when I installed version 0.5.1; the extension broke the ability to use the tab key to move from one form field to the next within a Web page. Upon reverting to version 0.5.0, all was well.
Oooh! This is great!