Mac: If you've been using Apple Notes on iOS or OS X, then you probably have a ton of stuff saved there. Technically, that should all be saved to iCloud, but if you want to do your own backup, or you just want to leave Apple Notes altogether, Notes Exporter is a tool for doing just that.
When you run Notes Exporter, it will scan your Notes, then pull everything out and turn them into plain text files. If you've used the new rich text notes in Notes for iOS 9 and El Capitan, Notes Exporter won't work for those, but otherwise it does the job. From there, you can either import the TXT files to another notes app or simply dump them on a cloud server for an extra backup.
Notes Exporter (Free)