[pluijzer]

If you are on Android I can recommend Orgzly. It ties in with Emacs' org-mode but is fully usable on its own with a sensible very convinient UI. It is open source and everything is saved in plain text that can optionally be kept in sync with Dropbox or WebDAV.

[pydry]

+1

It's the first and only note taking app I liked.

[LanternLight83]

As an alternative, Markor has worked great for my own flat-directory of org files.

[pydry]

I use that as well but it's better as a general text editor. It's unwieldy if you have a lot of notes and you can't attach dates/times/reminders.