I found this site recently: http://pilottutor.org . It offers free interactive lessons, and you don't even need to give your email address or sign up for anything. It is intended for private pilot and instrument students. I just tried the lesson on morse code---two thumbs up! Totally demystified the subject. Now, the creator of the lessons is not a CFI, so you should not use it in place of ground school, but what an excellent adjunct to flight training it is! This is a site I'm definitely bookmarking.