LEADING OPEN SOURCE DOCUMENT STORE
MongoDB (from “humongous”) is a scalable, high-performance, open source, dynamic-schema, document-oriented database. Written in C++, MongoDB features:
CouchDB provides a RESTful JSON API than can be accessed from any environment that allows HTTP requests. There are myriad third-party client libraries that make this even easier from your programming language of choice. CouchDB’s built in Web administration console speaks directly to the database using HTTP requests issued from your browser.
OTHER OPEN SOURCE DOCUMENT STORE
Apache Jackrabbit is a fully conforming implementation of the Content Repository for Java Technology API (JCR). A content repository is a hierarchical content store with support for structured and unstructured content, full text search, versioning, transactions, observation, and more. Typical applications that use content repositories include content management, document management, and records management systems.
RavenDB is a transactional, open-source Document Database written in .NET, offering a flexible data model designed to address requirements coming from real-world systems. RavenDB allows you to build high-performance, low-latency applications quickly and efficiently.
Data in RavenDB is stored schema-less as JSON documents, and can be queried efficiently using Linq queries from .NET code or using RESTful API using other tools. Internally, RavenDB make use of indexes which are automatically created based on your usage, or were created explicitly by the consumer.