https://soundcloud.com/subtersonic/modded-korg-monotron-demo-soundscape
Some of the mods:
MIDI input
9 volt power regulator for power adapter input
portamento
sub oscillator
square wave with variable pulse
switch to turn of oscillator
LFO with assignable destinations
LFO wave shape
Envelope generator and VCO
assignable destinations for envelope
integrated synthrotek pt2399 delay kit with modded delay time
It sounds really good!