A Themeable Scientific Calculator!
With 9 built-in themes and the ability to create your own, Flatcal is this the most versatile calculator on the Chrome Store. Don't forget to use alt-c to quickly open Flatcal! Best looking calculator or money back guaranteed (disclaimer: this is a joke, Flatcal is free).
P.S. FlatCal is open source! https://github.com/christianjuth/flatcal