Quick Graph

There are a lot of calculator programs out there, but Quick Graph is the first one I've run across that's not only free, but does a good job graphing, both 2D and 3D.  Although none of the images show it, it's also possible to graph more than one curve on the same set of axes.

Setting up a graph is as easy as typing in an expression.  The interface is pretty easy to use, and looks more or less like any other iPhone calculator.