Musictheory.net offers interactive lessons and training in music theory. Lessons include signature, chords, intervals, cadence and phrasing, and scales. After learning a part of music theory, you can practice it using the trainer interactive quiz tools. The site also includes utilities to calculate chords, generate customized staff paper, and generate a matrix. There is also an app version of these lessons, but it is not free.
Musictheory.net
