Raven DB is described as 'Raven is an Open Source (with a commercial option) document database for the .NET/Windows platform. Raven offers a flexible data model design to fit the needs of real world systems'. There are more than 10 alternatives to Raven DB for a variety of platforms, including Linux, Windows, Mac, BSD and Online / Web-based. The best alternative is MongoDB, which is free. Other great apps like Raven DB are Apache Cassandra (Free, Open Source), CouchDB (Free, Open Source), RethinkDB (Free, Open Source) and CockroachDB (Free).
MongoDB is a document database with the scalability and flexibility that you want with the querying and indexing that you need MongoDB stores data in flexible, JSON-like documents, meaning fields can vary from document to document and data structure can be changed over...
Apache Cassandra has become the leading NoSQL platform driving many of today's modern business applications by offering continuous availability, high scalability and performance, strong security, and operational simplicity while lowering the overall cost of ownership.
Apache CouchDB is a distributed, fault-tolerant and schema-free document-oriented database accessible via a RESTful HTTP / JSON API. It includes incremental map / reduce mechanics for queries and keeps data safe with an append-only datastore.
RethinkDB pushes JSON to your apps in realtime. When your app polls for data, it becomes slow, unscalable, and cumbersome to maintain. RethinkDB is the open-source, scalable database that makes building realtime apps dramatically easier.
Cockroach is a distributed key/value datastore which supports ACID transactional semantics and versioned values as first-class features. The primary design goal is global consistency and survivability, hence the name.
Couchbase is the NoSQL database market share leader, with production deployments at AOL, Deutsche Post, NTT Docomo, Salesforce.com, Turner Broadcasting Systems, Zynga and hundreds of other organizations worldwide.