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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.
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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.