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:Scientific modeBasic (large key) mode20 MemoriesKey click (haptic feedback)Continouous memory16-level stackFront four stack elements displayedMemories are not sacrificed to make room for program stepsRpnCalc 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