I just sync a directory called "Notes" everywhere and use whatever text editor is most comfortable on any given platform to edit Markdown documents. Helix on desktop, Markor (or, increasingly, Simple Text Editor) on Android. For checklists, same thing except I use todo.txt for the file format, and the todo script on the desktop and Simpletask on Android.
I have been looking for a self-hosted, concurrent collaborative web editor, as asking my wife to write Markdown is a bit much, and the syncing becomes more complex, but I haven't settled on something.