when I was a graduate student, I wrote tutor (http://github.com/mathgladiator/tutor ) which was a student-orientated compute algebra system. The idea was to give it a college algebra to calculus II problem, and it would spit out a right answer with a complete step by step explanation.

I actually had students using it, but I got distracted by my game engine.

If I go back to teaching, then I will probably make a start-up out of this.