Instacalc -- A Fast and Easy Web Based Calculator

Instacalc is a fast and fancy web based calculator that shows the answer as you type. It can be used also to generate charts, to perform metric unit and currency conversions and to share calculations with others.

clipped from instacalc.com Get real-time results. See what you're working on. Use readable numbers and equations.

It is even embeddable in a webpage:

via: InstaCalc: The Coolest Online Calculator so far | MakeUseOf.com