Wolfram|Alpha is the maker of Mathematica and more recently, Wolfram|Alpha apps for the iPad. The technology behind their products is designed to accept free-form input and serve as a knowledge and computational engine to produce and present results. They recently announced a series of Course Assistant Apps for Algebra, Calculus and Music Theory. The Algebra and Music Theory apps are $1.99 each and the Calculus app is $2.99. More here.