Pond, LilyPond

LilyPond just does that: it prints music in the best traditions of classical engraving with minimum fuss. Don't waste time on tuning spacing, moving around symbols, or shaping slurs. Impress friends and colleagues with sharp sheet music!

Well, for me LilyPond is more than that. It uses a markup language which allows another program to write musical notation. And that is exactly what we need as output for a music generator software.

For example, this code snippet:

{
c' e' g' e'
}

Produces this (as a pdf file):

Of course, one can produce more complex things, with chords, several staffs, lyrics, etc.
In order to generate MIDI output, you can just add a "\midi" tag in the LilyPond code. Just as easy as that!