Fantastic work. I've been learning music theory off and on to help with my guitar. I was hoping for a whole lot of interactive applications like yours, but found most lacking for me, though I did find a few on the iPhone: FretTrainer for muscle memory stuff. I figure out what to learn next based on Musicopoulus. Both are fair, each with quarks.

I've bookmarked you in my, just created, music "Toby" folder. Speaking of which, here's one last app recommendation that's totally unrelated: if you don't already have an extension for bookmark management on your computer, consider "Toby." It's a decent one for chrome, I use it as I have way too many normal bookmarks and finding stuff via scrolling becomes painful. I'd be interested if anyone else is using a different bookmark extension for Chrome as well.

Great job again!