There are a lot of NO-SQL solutions out there. So choosing one, can be a daunting task.
Kristof Kovacs made a comparison between of Cassandra, Mongodb, CouchDB, Redis, Riak, Couchbase (ex-Membase), Hypertable, ElasticSearch, Accumulo, VoltDB, Kyoto Tycoon, Scalaris, Neo4j and HBase.
So if you are interested in NO-SQL solutions, but need some help in finding one that fits your purposes, go read his blog post: http://kkovacs.eu/cassandra-vs-mongodb-vs-couchdb-vs-redis