There are a number of iPad apps that I use in the classroom either for teaching and demonstrating or for my students to explore the concepts.
A very important one is Playpad
I often will project it on the overhead screen and demonstrate a number of concepts for students to identify. For example, I'll play a kind of repetition and ask them to identify which one.
It is invaluable for demonstrating consonance and dissonance. I'll play SKIPS and STEPS and have students tell me if they are hearing a consonance or a dissonance.
Because it allows students to "play" the staff I use it in a number of ways:
1. Playing melodic shapes individually
2. Working in pairs they will perform melodies and harmonies where they are required to resolve dissonances.