Developer: Appcylon LLC
Price: $1.99 Download on the App Store
There are a number of tools to graph equations in the App Store, but the "Graphing Calculator" by Gabor Nagy has the dual advantage of being both excellent and free. As a math major myself, and a former user of programs such as "Mathematica", I was genuinely impressed with the interface and functionality of this tool. It's easy to enter equations, and you can overlay up to 4 single variable functions on top of each other, and specify range endpoints, color schemes etc.. The equations are fully zoomable by the usual iPhone gestures. An excellent addition to the "education" category.