GraphIt

the author of this app says:

GraphIt 2.0 is a fully-functional graphing calculator for PPCs and HPCs running Windows CE 2.0 or better. It supports standard scientific calculator functions, including absolute value, logn, and a wide range of trigonometric functions, including hyperbolic sine, cosine, and tangent! In addition to graphing any expression you can throw at it, it can also plot parametric curves (fun!), functions translated to the y-axis (i.e. functions in the form x=f(y)), and that's not all! It also includes the popular Polynomial Calculator, capable of performing lightning-fast operations on polynomials with any number of variables. Now you can factor polynomials with ease! It has every function you'd expect from a much more expensive graphing calculator, but only costs a fraction of the price, and works on the PPC you already own, so why not save yourself the cost of a Texas Instruments TI-81 or a Casio fx-7700.

Requirements

2.0 and Above.