I recently started a small synthesiser project, specifically to teach myself some SIMD:

https://github.com/seclorum/simdsynth

Its an 8-voice polyphonic synth with 1 oscillator per voice, a filter, an LFO, and EG's for the Osc and Filter.

I've been extremely happy with the performance I'm getting, running on the Raspberry Pi - in both Monome and Zynthian systems!

Monome and Zynthian are both game-changers for FOSS audio hackers. I have both running side by side and they are just a pure inspiration for audio exploration. It's very rewarding also, to be able to put my own code alongside all the other great stuff from both communities.