Apache Cassandra is a free and open-source distributed NoSQL database management system designed to handle large amounts of data across many commodity servers, providing high availability with no single point of failure.
Redis is an open source (BSD licensed), in-memory data structure store, used as a database, cache and message broker.
Apache CouchDB is open source database software that focuses on ease of use and having a scalable architecture.
MongoDB is a free and open-source cross-platform document-oriented database program. Classified as a NoSQL database program, MongoDB uses JSON-like documents with schemas