Not exactly retro in its build, but it might interest
folk hereabouts.
 
http://www.linusakesson.net/chipophone/index.php
 The Chipophone is an electronic organ whose guts have been replaced
 with a pair of microcontrollers to make it into a 1980s-style 8-bit
 synthesizer, for playing classic videogame chiptunes live. It helps
 that its creator is an excellent player of such things.
 For example, Rob Hubbard's /Spellbound/ from 1986:
 
http://www.linusakesson.net/chipophone/spellbound.php