I spent a day a couple of years ago writing an app to solve this kind of problem, and more. File format is plain text files stored in directories. Every directory becomes a tab; every file becomes an entry, with the first line in every file being its title. Along with plain text files, .log files are kept; these keep a log of every edit to the .txt file.

Nothing is ever deleted. No edits are ever lost; the whole undo stack is always there, and if you undo to a particular point and edit it, it just becomes a new revision on top of the old undo stack. There is no save / load - the app uses the directory it was started from, and saves when no edits have occurred for 5 seconds.

It's a combination of notepad, brainstorming log, todo list, and general notes. I keep it in sync across multiple machines with dropbox.