- Linnstrument is a polyphonic touch-sensing MIDI controller, built with Arduino Due. It's firmware is open-source and Geert Bevin shared some internals here.
- https://aionfx.com/kits/
- Pedalshield Due is an Arduino DUE Programmable Guitar Pedal
- https://www.rebeltech.org/product/owl-pedal-mkii/ (not sure if active)
You might find this YouTube channel helpful for DIY effect pedals.