A modular synthesizer! I designed the case in CAD, put it online for free, and it got kinda popular in the DIY synth community. Some companies are making it.

I built all modules from scratch with my own circuit designs too. Mostly novel designs, especially in oscillators and filters. A lot of JFETs being used to control voltages, amplitudes, etc. It was unstable and non-linear but had its charm :)