Flyclops, a game company, recently decided that MySQL wasn't cutting it for them anymore. They tried out a bunch of different databases including CouchDB, MongoDB, Cassandra, HBase, Neo4j, DynamoDB, PostgreSQL, and a bunch more. What they finally settled on was Riak.
Read all about their technical review process and how they ended up choosing Riak for their mobile multiplayer game.
Also, I'm downloading the free version of their game "Domino!" now to try it out. It looks like Words With Friends, or Letterpress but for Dominoes. Very nice.