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