MagicPlot calculator is a free expression calculator that calculates mathematical expressions. You can provide any expression, and also use variables in those, and MagicPlot calculator will calculate that.
The feature that I like best in this free expression calculator is that you can use variables in your expressions, and provide values for those variables, and MagicPlot calculator will calculate the expressions for you. So, you can just keep changing the values of variables to compute similar expression for different values. This is much better than Windows calculator(, or even replacement for Windows calculator).
MagicPlot calculator also comes with lot of built-in functions, like, sin, cos, atan2, and more. This gives you the flexibility to use these functions also in your expressions.
MagicPlot calculator does not even needs to be installed. Just download it, and start using it. It is completely portable. Still, it maintains a history of expressions.
Apart from that, MagicPlot free expression calculator comes with syntax highlighting, and parenthesis matching.