[antoncohen]

RocksDB, LMDB, and LevelDB are basically low level disk representations used by the databases that do things like provide network access, sharding, and replication. OpenLDAP (LMDB), MySQL (MyRocks), Bigtable (LevelDB-like), Riak (LevelDB), etc.

Many are or can be used as key-value stores. MySQL actually has a memcached compatible KV store, using InnoDB for storage. Postgres has HStore. A lot of the distributed databases roughly fall into the category of KV stores: HBase, Riak, Cassandra, DynamoDB, etc.