If you have any interest in web design, understanding how pages are put together and how big different aspects of pages should be is important.
MeasureIt is a tiny add-on for Firefox that adds an unobtrusive ruler icon the browser. When clicked, the page you're browsing will lighten and then you can click and drag out a ruler over any area of a page to get a precise measurement of how many pixels it covers.
If you're editing something like a Wordpress theme, and want to know how big various parts should be, pixels is the measurement of choice, and so MeasureIt is a useful tool. There is no way to automatically save the values you measure, although you can highlight the shown measurements and copy them to the clipboard, which is easy. The ruler won't scroll your page in any direction either, so you can only measure things within the area you are viewing.
MeasureIt is a simple little tool, especially handy for anyone who wants to get into web design.