CalcXpress

CalcXpress is a mathematical calculator which is more than just a calculator. Some of the unique features include:
1) Ability to evaluate algebra, trigonometry, complex numbers, unit conversions etc on the fly.
2) No network connectivity needed, completely standalone
3) Ability to use variables to store values temporarily.
4) More than enough functions to calculate almost anything.
5) Simple interface

You can see for yourself the magic of this little app in the examples section after installing it.