I haven’t used SimpleNote for a long time, the lack of markdown support made it too limiting for my purposes, but if you wanted a very simple and lightweight note taking app that was available on a few different platforms and the web then it was pretty compelling.
And now it is a bit more interesting because all the offical Simplenote client apps have been open-sourced.
So that covers Linux, Windows,
OSXmacOS, Android & iOS.
License is GPL v2.
The server is still closed but they mention in the comments that they are have it in their roadmap for the future. I guess that probably covers the web-app too, but I guess you could probably pull most of it from the source for the Windows/Linux client (it’s an Electron app) if you wanted.
It wouldn’t surprise me to see people reverse engineering their own servers from the client source.