Kalkules is a portable free scientific calculator.
The program opens like a marginally expanded version of Windows Calculator, with a numeric keypad, basic operators and a good selection of more advanced functions and controls.
As usual, you can click buttons individually, or just type an equation and see the result appear when you hit Enter. (If you prefer the second approach then you can hide the buttons to minimise the interface, and maybe set it "always on top" to ensure the program is always available.)
Kalkules gets more interesting as you explore the menus and find its more advanced features. Click View > Variables, for instance, and a simple table appears where you can enter variable names and their values. Set x to 188.8.131.52 here, and you can then type x to represent that value in the main calculator.
A "Tools" menu provides various extra functions which might justify the download all on their own: function graphing, a unit converter, individual calculators for series summations, quadratics, polynomials and more.
Kalkules' doesn't have quite as much power or as many options as the high-end competition, but this keeps it relatively easy to use, and there's still more than enough functionality here for most people. A good mid-range scientific calculator.