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.