GMP is a free library for arbitrary precision arithmetic, operating on signed integers, rational numbers, and floating point numbers. There is no limit to the precision, except for the ones implied by available memory. It has a rich set of functions, and the functions have a regular interface. It is designed to be as fast as possible, both for small operands and for huge operands. The speed is achieved by using fullwords as the basic arithmetic type, by using fast algorithms with carefully optimized assembly code for the most common inner loops for a lot of CPUs, and by a general emphasis on speed (instead of simplicity or elegance). It is believed to be faster than any other similar library. The advantage increases with the operand sizes for certain operations, since GMP in many cases has asymptotically faster algorithms.
MathLib is a PalmOS shared library which makes a complete set of IEEE-754 double-precision math functions available to other Palm applications. These functions include all the standard routines normally accessed by including "math.h" on other systems, including trigonometry (with inverse and hyperbolic), logarithms, exponentiation, and miscellaneous helper functions.
QPlot is an Advanced Matematical Calculator for Qtopia, featuring a nice calculation mode (you see the statement and the result), arbitrary precision calculations, an extensive math library (trig, inverse trig, hyp, inverse hyp, algebraic, etc.), integration, differentiation, quadratic differentiation, graphing (including the ability to easily move around and zoom in/out), equation solving (quadratically or using Newton's method),an ability to write programs (and run them directly from desktop), and extensibility.