CustomCalc is a powerful totally customizable calculator. It lets you place the function buttons that you use most right where you prefer them to be, so you can execute them in just on tab, or click.
Input mode may be RPN, or Algebraic. It has all the Math and Trigonometric functions, an advanced set of Financial functions with powerful worksheets, Statistical functions for the analysis of data series for up to two variables, and performs Linear Regression, Date and time calculations, and much more.
ZDNet rating: (5 stars) "an impressive customizable calculator...extremely easy-to-use"