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.

UDPATE on 11/04/2025: I've had to stop developing my Simple Calculator in Rhino because the NetBeans configuration files which are necessary to compile the Java code have been lost when I had to reinstall my operating system. The Java code is still available on GitHub, but it's no longer compatible with the new JavaScript code. Please use the Arithmetic Expression Compiler instead, especially if it's important to you to target i486 with the assembly code it produces. Besides, I received some validation that I am doing good work for my PicoBlaze assembler and emulator (people from other countries joining me and adding new features), whereas I haven't received any validation that my Simple Calculator or the AEC-to-x86 compiler are useful to anybody, so I will rather work on my PicoBlaze assembler and emulator than on those things.