MorphEngine is a calculator engine.
It powers ND0 and ND1, calculators for iPhone and iPod touch, and the upcoming CalcPad for iPad.
It's "type-rich" and supports the same data types as an HP-49/50g calculator, and more. There's the concept of a stack, RPN for an operational mode, and RPL. Its functionality is a mix of HP-28, HP-49 and new tech, with an eye toward full 50g compatibility in the future and many more unheard-of calculator features.
More information about MorphEngine can be found at http://naivedesign.com/ND1/ND1_Reference.html.