Cassandra is a highly scalable, eventually consistent, distributed,
structured key-value store. Cassandra brings together the distributed
systems technologies from Dynamo and the data model from Google's
BigTable. Like Dynamo, Cassandra is eventually consistent. Like
BigTable, Cassandra provides a ColumnFamily-based data model richer
than typical key/value systems.

Cassandra was open sourced by Facebook in 2008, where it was designed
by one of the authors of Amazon's Dynamo. In a lot of ways you can
think of Cassandra as Dynamo 2.0. Cassandra is in production use at
Facebook but is still under heavy development.

WWW: http://cassandra.apache.org/