RpnCalc is the best and most popular RPN calculator on the Android Market.
It has an interface that users of the HP calculator line will be completely at home with, including these features:
Basic (large key) mode
Key click (haptic feedback)
16-level stack (configurable)
Front four stack elements displayed
Memories are not sacrificed to make room for program steps
RpnCalc has a sixteen-level stack to hold more data. The front four elements on the stack are visible at all times, making it much easier to keep track of where you are in your calculations.
See http://www.efalk.org/RpnCalc/ for manual
Version 2.23 fixes a minor display error