9. CloudKit

CloudKit provides schema-free, auto-versioned, RESTful JSON storage with optional OpenID and OAuth support, including OAuth Discovery.

Home: http://getcloudkit.com/
Quick Start: http://getcloudkit.com/api/
Download: https://github.com/jcrosby/cloudkit

10. Perservere

Persevere is an open source set of tools for persistence and distributed computing using an intuitive standards-based JSON interfaces of HTTP REST, JSON-RPC, JSONPath, and REST Channels. The core of the Persevere project is the Persevere Server. The Persevere server includes a Persevere JavaScript. client, but the standards-based interface is intended to be used with any framework or client.

Home: http://code.google.com/p/persevere-framework/
Quick Start: http://code.google.com/p/persevere-framework/w/list
Download: http://code.google.com/p/persevere-framework/downloads/list

11. Jackrabbit

The Apache Jackrabbit™ content repository is a fully conforming implementation of the Content Repository for Java Technology API (JCR, specified in JSR 170 and 283). A content repository is a hierarchical content store with support for structured and unstructured content, full text search, versioning, transactions, observation, and more.

Home: http://jackrabbit.apache.org
Quick Start: http://jackrabbit.apache.org/getting-started-with-apache-jackrabbit.html
Download: http://jackrabbit.apache.org/downloads.html

Conclusion
Document databases store and retrieve documents and basic atomic stored unit is a document.  As always your requirement leads into the decision. You need to think about your data-access patterns / use-cases to create a smart document-model. When your domain model can be split and partitioned across some documents, a document-database will be a suitable one for you. For example for a blog-software, a CMS or a wiki-software a document-db works extremely well. But at the same time a non-relational database is not better than a relational one in some cases where  your database have a lot of relations and normalization.

Just check the following link from stackoverflow also to cover the pros/cons of Relational Vs Document based databases.