UPDATE on 30/4/2019: If you like this calculator, perhaps you should download the desktop version of it, which also has more features. I built it using the Rhino framework (developed by Mozilla), which allows me to use Java classes from JavaScript, without having to learn a lot of Java, making it relatively easy to make feature-full GUI programs. You can download it here (and please download the latest version of Java Runtime Environment, I have no interest in dealing with the Java equivalents of Internet Explorer 6).
The feature that distinguishes it from other calculators is the ability to convert arithmetic expressions to assembly, particularly assembly that's easy to embed into other programming languages. You can see an example of how to embed it into C here. Or, maybe even better, watch this video.