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.

For more information see http://incubator.apache.org/cassandra