Try the Acron RPN Calculator before you buy. The lite version has the following limitations:
- Answers are always displayed as floating point. For instance, the full version knows that the sin(45) = sqrt(2)/2, but the free version only knows that it is 0.707106...
- Functions that cannot use floating point numbers as input have been disabled. This includes factor, gcd, lcm, factorial, and all of the binary/decimal/hexidecimal integer functions.
- Macros, in-line editing, and statistics are not available in the free version.

Check out our tutorials at http://www.acrongames.com/rpncalculator.html