Couchdb-history is a project mainly written in PHP, based on the Apache-2.0 license.
Stores each revision of a document in its own document.
People's first impression of CouchDB's _rev attribute in its documents is that they get built in management of their data's history, much like git or svn give them. Then they find out about compaction and MVCC.
This tool meets that need for them by creating a separate document for each revision of a document, while allowing them to interface with their data like normal.
The project is under the Apache License, which you can find in the LICENSE file.