Redis, developed in 2009, is a flexible, open-source, key value data
store. Following in the footsteps of other NoSQL databases, such as
Cassandra, CouchDB, and MongoDB, Redis allows the user to store vast
amounts of data without the limits of a relational database.
Additionally, it has also been compared to memcache and can be used,
with its basic elements as a cache with persistence.
http://redis.io/documentation