Grapher is a math app, with an easy to use interface and features like 2D and 3D graphing, a function based calculator, a numerous graph functions, and even a small scripting language. The package includes the program, example scripts and an HTML guide to using it. You may also want to check out Crystal Math, which is a complete rewrite of Grapher to allow for far more power and flexibility. Currently it is very usable, but quite far from complete (the interface looks almost exactly like Grapher's Calculator, but the core is vastly different, and it' cannot graph functions yet).
Screen Shot - 2D GraphScreen Shot - 3D GraphScreen Shot - The Calculator