MemcacheDB is a distributed key-value storage system designed for persistent. 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 completed), so any memcached client can have connectivity with it. MemcacheDB uses Berkeley DB as a storing backend, so lots of features including transaction and replication are supported.
reddit.com initially used MemcacheDB, but then switch to use Cassandra.
http://blog.reddit.com/2010/03/she-who-entangles-men.html
http://www.reddit.com/r/programming/comments/b81v1/i_was_hoping_we_could_get_a_good_technical/
http://www.reddit.com/r/programming/comments/bcqhi/reddits_now_running_on_cassandra/