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.
Virtual MIDI Piano Keyboard is a MIDI event generator and receiver. It doesn't produce any sound by itself, but can be used to drive a MIDI synthesizer (either hardware or software, internal or external). You can use the computer's keyboard or mouse to play MIDI notes. You can use the Virtual MIDI Piano Keyboard to display the played MIDI notes from another instrument or MIDI file player.