undohist

Persistent undo history for GNU Emacs

Usage

This extension provides a way to use undo history of individual file buffers persistently.

Write the following code to your .emacs:

(require 'undohist)
(undohist-initialize)

Now you can record and recover undohist by typing C-x C-s (save-buffer) an C-x C-f (find-file). And then type C-/ (undo).

Note that this package is no longer actively maintained.

undo-fu-session is an updated version of this package, with support for undo/redo and compression among other improvements.
undo-tree is an undo system for Emacs, which includes support for restoring an undo-session from saved files.