I have two suggestions for the app project that I feel most math instructors can make use of especially the first one and the second one could be also used for general public as well as math faculty.

1) I thought it will be usefull to have an app where we can enter the equation of a curve and a point where we would like to draw a tangent line to this curve at. The app that I will find very useful in my calc class, should first show several secant lines drawn to this curve around the point we entered and then draw the tangent line to the curve. At the end of all this an animation of all these together i.e. the secant lines and the tangent line would help tremendously with the derivate understanding of the students. A visual tool like this one communicates the idea of the derivative much better than the limit definition I put on the board.

2) My second suggestion is an app that calculates where best to sit in a movie theater. I have the computations and the needed parameter research part of this project typed up already I'll be happy to e-mail it if there is any interest. Briefly; the app programmer will calculate the best seat in a theater by maximizing a viewing angle. I thought I could use such an app to increase the understanding of certain trig methods in my trig or precalc class. In my precalc class I have shown briefly how the students can get the formula for the viewing angle depending on where you sit in the theater. But that is as far as we were able to go I'm hoing the app could do the rest for us i.e. either graph the viewing angle formula or differentiate it and give us the best seat in the house along with a graph or a derivative equation. Using these I can eleborate further mathematical ideas at play.