midi

MIDI (Musical Instrument Digital Interface) is the most commonly used specification for interfacing both digital and analog instruments to digital devices. To learn more about MIDI, see Google.

Here is a cool MIDI annotation trick I learned from blader on Google Answers (here):

1. Do a Google search on "diagnosis murder" midi to find diagnosis-murder.mid.
2. Do a Google search on midi notation software to find notation.com.
3. Run Notation Software's MidiNotate on the midi file to produce diagnosis-murder-1.gif and diagnosis-murder-2.gif.

GNU LilyPond prints beautiful music. (Link from Karl.) See also Mutopia.