Not a calculator in the physical sense (however it's possible...) but instead a scientific calculator that uses the serial monitor as an interface. Instructables user "RayBurne" has used the IEEE floating-point libraries to create a sketch which offers a wide variety of functions, and can also be modified to add your own. We took it for a spin earlier and it works well, however there's no guarantee to the accuracy:
Nevertheless - give it a try for yourself. For more information and the sketch, click here. And you'll need the "streaming" library to compile the sketch. And for more, we're on twitter and Google+, so follow us for news and product updates as well.
If you're new to Arduino, the first step is a solid board for your projects - our Freetronics Eleven - the Arduino-Uno compatible with low-profile USB socket, onboard prototyping space and easy to view LEDs: