Org mode is very dear in this space (to us Emacs users anyway). I'm an iPhone user and was missing quick access to my content (view/add/edit) while on the go. Org is a rich markup, but I've been slowly chipping at my own iOS app with https://plainorg.com.
Compared to markdown, our non-Emacs org options are few but here are others I know about:
Compared to markdown, our non-Emacs org options are few but here are others I know about:
https://beorg.app
https://braintool.org
https://flathabits.com (I author this one too)
https://organice.200ok.ch
https://orgro.org
http://orgzly.com
Karl Voit also has a great effort going to advocate org outside of Emacs and documents some of the tools at https://gitlab.com/publicvoit/orgdown/-/blob/master/doc/Tool...
edit: formatting