MemcacheDB is a distributed storage system designed for persistence.
It is not a cache solution, but a persistent storage engine for fast and
reliable key-value based object storage and retrieval. It conforms to
memcache protocol (not complete), so any memcached client can connect to
it. MemcacheDB uses Berkeley DB as a storing backend, so lots of
features including transaction and replication are supported. 

WWW: http://code.google.com/p/memcachedb/