I use Notebooks[1] on iOS. The big draw of this application for me is the ability to synchronize via WebDAV. I do not want to store my notes on any 3rd party server, but want to be able to access them on mobile devices and this is the compromise that I found.

On Mac OS X, I use nvALT[2] setup to use the same directory.

The directory itself is inside EncFS filesystem, which is mounted over cyphertext directory inside Dropbox so there is continuous encrypted sync as well.

[1] http://www.notebooksapp.com/ [2] http://brettterpstra.com/projects/nvalt/