How many database systems there are that use JSON for storage or transport? I know of:
I remember I saw yet another vendor in a SO user's profile. That systems was using what they called binary JSON, but I can't remember the name of the product.
Lately, it appears that more and more DB projects are looking towards JSON for persistence storage. Some of them are even embracing HTTP as a transport layer.
MongoDB does not use JSON but BSON.
RethinkDB is a document oriented database with a JSON data model, which supports sharding and replication
Persevere is another DB that is loaded up with JSON:
http://www.persvr.org/
Primary transport is JSON HTTP/REST
Data is stored as JSON
It has a native JS/JSON client for running directly from the browser.
If you want a schema then it is defined using JSON
Take a look at Basho's Riak. It has a number of things in common with CouchDB: Erlang-based, Javascript MapReduce API, HTTP transport, JSON document format and multi-master replication. It doesn't aim to be quite as simple as CouchDB (CouchDB is more "opinionated"), but they give you a lot of options for adjusting CAP parameters to meet the needs of your application, per-write.
This is a non-authoritative list.
Source: Wikipedia - Document Oriented Databases
If you want to store your JSON document in AppEngine's Datastore, you can have a look at Ubud-db, https://bitbucket.org/f94os/ubud-db/wiki Ubud-db is a Document store on AppEngine with a REST-JSON API.
MongoDb is the one that uses a binary JSON storage format. I don't know if there is another that is document oriented. Most of the others are key value stores and can only retrieve an object based on one key.