RethinkDB is an open-source distributed json database. It scales to multiple machine using very little effort. RethinkDB have very good features including distribute JSON document with query language (play with query language), help to control/setup the cluster and many more. RethinkDB is very easy to setup and learn.
ReThinkDB URL http://www.rethinkdb.com
ReThinkDB Installation http://www.rethinkdb.com/docs/install/
ReThinkDB Quick Start http://www.rethinkdb.com/docs/quickstart/
ReThinkDB Examples http://www.rethinkdb.com/docs/examples/
ReThinkDB API (Python) http://www.rethinkdb.com/api/python/
ReThinkDB API (Ruby) http://www.rethinkdb.com/api/ruby/