rpnCalc 1.6.1 for Mac could be downloaded from the developer's website when we last checked. We cannot confirm if there is a free download of this app available. The application lies within System Tools, more precisely Dashboard Widgets. This free Mac application was originally designed by Rick Lansky. According to the results of the Google Safe Browsing check, the developer's site is safe. Despite this, we recommend checking the downloaded files with any free antivirus software.
From the developer:
rpnCalc is a reverse polish notation (RPN) calculator. The stack behavior is based, rather loosely, on the HP-28S. rpnCalc offers commands which allow the stack values to be copied onto the clipboard. This functionality makes use of the pbcopy system-level command.
You may want to check out more software for Mac, such as RPNcalc-11c, RPNcalc-12c or RPNcalc-15c, which might be similar to rpnCalc.