A good review of the various NoSQLs products for
1) supporting multiple datacenters,
2) the ability to add new machines to a live cluster transparently to the your applications,
3) data model,
4) Query API and
5) Persistence Design.
The systems reviewed are: Cassandra, CouchDB, HBase, MongoDB, Neo4J, Redis, Riak, Scalaris, Tokyo Cabinet and Voldemort.