While I’m not very much into mathematical calculations and the windows calculator works just fine for me, I know that students doing research online, especially engineering students would love to use an advanced online calculator which has better functions than the windows calculator.
eCalc is a free calculator which has both the basic and the advanced versions. It’s browser based and easy to work with. It’s scientific calculator has advanced features like support for complex numbers, unit conversion, base conversion, and has a built in equation solver. Those interested in using this should first watch the demo video which gives a good explanation of the advanced features and how to use them.