Scheme Note is a Scheme-like music file format and MIDI conversion tool. It takes a text file, which is a representation of a song, loosely tied to a MIDI-like event structure, and outputs a corresponding MIDI file. It is hand-editable for simple songs, and should be very easy to parse for a GUI.
Mutabor is a program that allows you to perform and compose microtonal music using a normal MIDI keybord and normal MIDI equipment. Its powerful description language is intended to allow permanent retunings and complex tuning logic. One application is just tuning. Mutabor has been used for this purpose to make available all pitches that can be reached by stacking perfect thirds and perfect fifths. The stable version 3.0x is available in German only and runs under Windows and Wine. The unstable branch can be accessed via CVS (but build bugs should be expected) or downloaded from the Berlios developer site.