The Windows standard calculator is usually enough for me. However other people may have more advanced needs when it comes to mathematical calculations.
WiseCalc is a complete scientific calculator with which you can do many different operations covering not only basic arithmetic but also more advanced calculations involving physics constants, mathematical functions and more.
The good thing about WiseCalc is that it doesn't work only as a calculator, but also as unit converter with support for many different types of measurements. Also, you can use the calculator either with your keyboard or with the program's virtual keyboard.
WiseCalc features a fairly complete configuration menu and also allows you to define your own constants, functions and units to add them to the program's options.
WiseCalc is a very complete scientific calculator that some people may find complicated but that will be very useful for advanced users.