Arguably the Web's single most popular sound format, the MIDI file is everywhere these days. A small data file, a MIDI file contains a musical sequence that can drive your sound card or electronic musical instrument if it has General MIDI (GM), General Standard (GS) or Extended General MIDI (XG) compatibility. (Virtually all sounds cards do). If you've ever encountered a Web page that featured background music, odds are you're listening to a MIDI file. The same goes for if you've ever received a digital postcard. From classical music to rock 'n' roll to techno: no matter what your musical tastes, you can find your favorite tunes in MIDI format somewhere on the Web if you look hard enough.