JSCalc is a bookmarklet that creates a calculator on top of any web page that you are currently viewing. Just activate the bookmarklet and start typing in the figures that you need to calculate – add up prices, figure out percentages, determine the 24th decimal of Pi; anything you can count.
Right-click here and "Bookmark" JSCalc to access it anywhere, or click the link to try it out.
Note: due to Internet Explorer's restriction upon URL lengths this bookmarklet will not work in that browser. It is only guaranteed in Mozilla or Firefox.