The Shruthi by Mutable Instruments is maybe the most known DIY-synth. It’s a hybrid monosynth, combining the grittiness of a digital synthesizer with the warmth of real analogue filters.
After building this the Shruthi in 2012 I found out about the Shruthi-XT:
The controller extension was developed and designed by Frank Daniels who can be found at the Mutable-Instruments forum and as far as I know it was the very last one. It was a difficult and time consuming build but I learned so much from it.
In the meantime Olivier from Mutable-Instruments has also released his own Shruthi-XT.