WP 34s is the most powerful and complete implementation of a classic RPN scientific calculator. Now available on iOS too (see the support site for other emulators and for the real stuff).

The function set of the WP 34S is based on the most powerful programmable RPN calculators built so far and we expanded the set, incorporating programmer functions (base 2, 8 & 16), a fraction mode, probability distributions, a stopwatch...
We also included numerous additional useful functions for mathematics, statistics, physics, engineering, programming, I/O, etc., such as:

+ Euler’s Beta and Riemann’s Zeta functions, Bernoulli and Fibonacci numbers, Lambert’s W, the error function, and the Chebyshev, Hermite, Laguerre, and Legendre orthogonal polynomials (no more need to carry heavy printed tables),
+ many statistical distributions and their inverses: Poisson, Binomial, Geometric, Cauchy-Lorentz, Exponential, Logistic, Weibull, Lognormal, and Gaussian,
+ programmable sums and products, first and second derivatives, solving quadratic equations for real and complex roots,
+ testing for primality,
+ integer computing in fifteen bases from binary to hexadecimal,
+ extended date and time operations and a stopwatch3 based on a real-time clock,
+ financial operations such as mean rate of return and margin calculations,
+ 88 conversions, mainly from old Imperial to universal SI units and vice versa,
+ 50 fundamental physical constants as accurate as used today by national standards institutes such as NIST or PTB, plus a selection of important constants from mathematics, astronomy, and surveying,
+ Greek and extended Latin letters covering the languages of almost half of the world’s population (upper and lower case in two font sizes), plus mathematical symbols.

A beginner manual can be donwloaded at http://sourceforge.net/projects/wp34s/files/doc/WP_34S_Beg_Guide.pdf and a full, very complete version at http://sourceforge.net/projects/wp34s/files/doc/Manual_wp_34s_3_1.pdf