Remember your first Casio, where you spent hours pretending you were D.J. Jazzy Jeff? Okay, maybe that was just me. Well, now you don’t need an electronic organ to make like a DJ: This site lets you choose from more than 125 sounds to make your own song. You can also change octave, key, and tempo, or even create two different melodies at once. The magic is created by fractal music, an emerging field that uses geometry and algorithms to generate melodies. But you don’t have to worry about the technology; just log on and get your groove on. (