NoSQL [Not Only SQL] database is an alternative to traditional RDBMS systems like SQL Server, Oracle and it is having simple, lightweight mechanism for storage and retrieval of data that provides higher scalability and availability than traditional relational databases. NoSQL databases are highly used in Big Data systems, where the data's nature does not require a relational model and most of the data is semi-structured or unstructured.

RavenDB is a transactional, open-source Document Database [NoSQL] 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. RavenDB stores data schema-less as JSON documents within the database, and can be queried using LINQ queries or using RESTful API.
Raven DB, Mongo DB are few of them providing NoSQL Databases.

For more details, please refer:

http://ravendb.net/
http://blogs.technet.com/b/sateesh-arveti/archive/2013/09/12/look-at-raven-db-integration-nosql-with-aps-net-mvc-4-application.aspxhttp://blogs.technet.com/b/sateesh-arveti/archive/2013/09/12/look-at-raven-db-integration-nosql-with-aps-net-mvc-4-application.asp