Redis - Redis is an open source in-memory data structure project implementing a distributed, in-memory key-value database with optional durability.
memcached - High-performance, distributed memory object caching system
Hazelcast - Clustering and highly scalable data distribution platform for Java
CouchBase - Document-Oriented NoSQL Database
MongoDB - MongoDB (from "humongous") is a scalable, high-performance NoSQL database.
Apache Cassandra - The Apache Cassandra database is the right choice when you need scalability and high availability without compromising performance.