AddUp is a free calculator able to process non-trivial arithmetic expressions such as: -(((1 + 2) * 3)^4)% + 56e-7 / 8

Unlike regular desk calculators where the numbers and operators you provide disappear when you enter the next one, AddUp retains all this information for review in an editable work area. Expressions can be edited and re-calculated if necessary. Results can be saved to a text file.