One of the absolutely coolest open source project I have come across recently was #synthstrom open-sourcing the OS of their music making machine, the Deluge. The #deluge / #synthstromdeluge is in my opinion still the best electronic music making machine there is.
Now there is a community firmware at https://github.com/SynthstromAudible/DelugeFirmware, next to the official one and the ways they've built on the hardware platform is just incredible.
It's one of the best cases for #opensource I've yet to see and I feel like there are a lot of lessons to be learned from it. I'm not that qualified to comment on them, but I think it is interesting that the switch from closed to open source for this hardware platform happened quite late, after the Deluge had built a dedicated fanbase over years and was already a very solid machine. Also the company has stuck to the same hardware platform now forever, which is rare, but super cool.
I do not know how many Deluge/ #synth fans there are in the fedi, but I finally got to play around more in-depth with the new community firmware over the last couple of days and the new Grid view (Ableton inspired) is amazing.