Recs.
Updated
RethinkDB is built to store JSON documents, and scale to multiple machines with very little effort. It has a pleasant query language that supports really useful queries like table joins and group by, and is easy to setup and learn.
SpecsUpdate
Pros
Pro Powerful query language
RethinkDB's ReQL is a very powerful functional query language. The functional aspects of ReQL and the straightforward implementation of the Node driver for Rethinkdb make it a natural fit for Javascript developers. You no longer have to type some obscure syntax in quotes (aka SQL), your queries are just "natural" Javascript functions in the same way you would use lodash to handle your collections.
Pro Ease of cluster setup
You can directly tell it to shard/replicate and how many shards/replicas depending on the amount of nodes. Each node doesn't need anything except one other node's ip/port in the cluster to join and maybe the auth.
Cons
Con No user accounts
This is just the database, you need to setup your own auth and user accounts (such as using Auth0).