https://www.hiroapp.com - Note-taking reduced to the core that just works™. Offline first, no signup needed, easily sharable (url, sms, email) and (web)realtime sync between all devices and collaborators.

My co-founder and I moved on to a new project a year ago, but this thing is still buzzing along on a cheap DO box and works like a charm with basically zero maintenance. Frontend is vanilla JS, backend in Go and the protocol is our slight modification of differential sync[0] to (re-)synchronize all text and metadata.

[0] https://neil.fraser.name/writing/sync/