Hmm… Well, I’ve dealt a bit with MIDI, but mostly input. Built a synth with three SID chips way back, and before then, I was playing around with my own trackers and other audio code on the Amiga. Oh, and a small game I did as a university project could actually send MIDI to a hardware synth for sound effects, so I guess you could think of that as a MIDI controller of sorts…
Last controller project would be my desk with integrated Fatar keybed, but that’s basically just a straight transplant from my old SL880 master keyboard.
Currently, I’m trying to get around to finish my analog triple piano pedal setup, where I’m using a Roland RPU-3 hooked up to a box with a Doepfer “Wheel Electronic” module in it.
Apart from modular synth stuff, Dopefer offers a bunch of handy modules if you want to deal with MIDI and various sorts of hardware without building custom electronics.