For synth heads, there is also a well-regarded Syntorial app/course. (BTW, has anyone tried their Building Blocks? There are very few online reviews.)

Building Blocks was a pack-in when I bought Syntorial in last years sale. It's technically different (web-based instead of a standalone app like Syntorial), and tries to do with music theory as a whole what Syntorial does with synthesis. Since it goes through the same 'theory, practice and create' cycle every time (eg. once for every chord type) I found it getting a bit tedious after a while.