Guitar pedals - as @Dud suggested you can adapt them into rack modules when DIY, but remember to use this module before - AI006 Stomp Adapter Schematic Help
http://www.generalguitargadgets.com/how-to-build-it/technical-help/schematics/
https://www.diystompboxes.com/pedals/schematics.html
And my link collection that is still growing and overwhelming:
http://www.yusynth.net/Modular/index_en.html
https://web.archive.org/web/20171111221410/http://www.cgs.synth.net/modules/index.html#cgs03 -
http://www.skullandcircuits.com/resources
http://www.jt30.com/potfix/index.html - pot fixing - if you need logarithmic, but have only linear - not 100% accurate, but still, better than nothing
https://www.nonlinearcircuits.com/wiki/doku.php?id=modules:list
https://web.archive.org/web/20171111221410/http://www.cgs.synth.net/modules/index.html#cgs03 -
https://www.stereoping.com/diy-eurorack/?lang=en - reverb seems to be interesting, but I do not see any schematics : (
http://www.solorb.com/elect/musiccirc/panner/ - Interested project how to make stereo panner, seems to be usefull for drones/ambient music
Based on Arduino:
https://hackaday.io/project/161396/ - FatCat- Gorgeous climate, but setting seems to be complicate - 45min tutorial movie on YT, check guys channel
https://bleeplabs.com/product/bleep-drum-midi/ - Blep Drum
http://familjenronnberg.se/~niklas/diy/eurorack/bleep_drum/ - Version of the Blep Drum
https://hackaday.io/project/164521-drumkid-aleatoric-drum-machine - DrumKid - latest version have MIDI IN and OUT, I will build it using touch keys instead standard buttons - will post the results soon
Circuit bending - use toys/simple devices and make crazy sound of them
https://synthvibrations.wordpress.com/tag/casio-sa/ - Casio SA mods
https://soundbender36.wordpress.com/2017/12/19/casio-sa-38-circuit-bending/ - Modded SA
https://www.papernoise.net/concertodrone/ - Well-documented project, see PDF
https://hackaday.com/tag/circuit-bending - weird projects
http://getlofi.com/category/diy-synth/- weird projects #2
https://www.instructables.com/id/Lo-Fi-Voice-Sampler-Circuit-Bending-/ - 1$ circuit bending module to loop 10s sound