The MeasureIt Firefox plugin is a simple little tool that allows you to measure the size of elements in your browser.

After installing MeasureIt, an icon will show up in the lower left hand corner of your browser:

When you click the icon, your cursor will turn into a crosshair icon. You can then draw boxes around anything on the page and MeasureIt will highlight the selected area and display the dimensions:

It's just that simple! My previous way for measuring something on a page was to take a screenshot and then use MS Paint. You can see why this is saving me time.