SuperJam
NOTE: THIS SOFTWARE APPEARS TO BE ABANDONWARE. THERE IS A GOOD FAQ, HOWEVER.
SuperJAM! (or SJam) is a MIDI "jamming" program. The idea is that, rather than composing and arranging all the parts of a musical work, the user can give some basic information to the program -- chords, musical style, general tone or personality of the music, what instruments to use -- and the program will handle the details, generating complete arrangements in real time. The user can modify the arrangements at any level, add a melody, print sheet music using the included Notation Station, and export the song as a MIDI file.