TuneHopper
While this app is aimed at the younger crowd there’s absolutely no reason it wouldn’t be fun for adults. Why not start your education with something so fun you don’t even know you’re learning?!
Music Theory for Beginners
This is a good little app that will take you through the basics of music theory. Don’t know what music theory is? This is a good place to start. While it won’t replace practice on your actual instrument of choice it will give you the background to become a much better musician.
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Music Theory 101
A basic app that doesn’t cost too much. Not nearly as easy to use or interactive as the others.