Professional music typesetting, according to the book "The Art Of Music Copying" by Clinton Roemer. Reads MusicXML, CMN, ABC and Guido. Produces MIDI, EPS, PDF, PNG and SVG; from small images for the web to full printable sheet music.
Yet another human-readable plaintext music notation language. Somewhat similar to ABC and also Guido. Empty lines divide blocks of music, and every line is for one voice. Blocks can be repeated and merged to simplify structured composing.