Ernest Brock has GraphNCalc83 Color Graphing Calculator for the iPhone, iPod Touch, and iPad in the iOS App Store for FREE (regularly $5.99) for a limited time.
Texas Instruments also has its TI-Nspire Computer Algebra System (CAS) for the iPod Touch in the iOS App Store for FREE (regularly $29.99) for a limited time.
GraphNCalc83 uses touch, color and blazing speed to handle your math problems. The familiar calculator keyboard, arrow keys and functions are enhanced with full touch input for all graphs, editors and menus. Enter calculations in natural textbook format. Enter fractions and view result as fractions. Trace graphs with your finger. Zoom in and out with a pinch. Explore regressions with a tap on the screen. Scroll through the list, matrix and calculation history with a swipe. Tap the ? key for help. Written using the latest 64 bit math libraries, GraphNCalc83 harnesses the amazing computing resources of the iPhone and iPad to provide a colorful, fast and fluid experience. Graphs scroll, glide and zoom smoothly. Matrix, statistics and BASIC programs run 500x times faster than traditional calculators.