Aquallegro is a great way to improve your musical ability. It contains quizzes on various aspects of music, as well as detailed help files with tips and mnemonics. All of the quizzes are scored while in progress, so the user continuously knows how well they're doing, and it is easy to customize Aquallegro so it works the way you want it to. It also contains Administrative features for teachers, making it an excellent tool in any music program.
Aquallegro is perfect for the comprehensive music student or just for the curious that want to hone their musical skills. With a price of $0, how could one ask for more?