I use Org files, which are basically simple plain text files. The Org syntax
is very simple and obvious. They fully readable by people who don't know the
format.
Because they are simple plain text files, you can:
Write them with any text editor, even if it doesn't know the format
Easily search through all of them with common programs
Easily make backups with common programs
Use common revision control systems
Replace strings in all of them at once
Send them to others by just copy-pasting the text
