Ape, the Adaptable Persistence Engine, adapts Python objects to storage on the filesystem, in relational databases, or practically anywhere else. It combines the advantages of transparent object persistence with arbitrary databases and formats. It is especially designed to work with ZODB, but can work independently of both Zope and ZODB.
http://hathawaymix.org/Software/Ape/
Not actually a ZopeProduct, but put it here for now. See also DirectoryStorage.
APE has recently been used to run Zope on top of a Subversion repository - see developer Mail lists 2004/04
APE's author has deprecated it as of 2006 - things got too complex.