Datomic - The fully transactional, cloud-ready, distributed database
Redis - Redis is an open source in-memory data structure project implementing a distributed, in-memory key-value database with optional durability.
Datahike - A durable datalog database adaptable for distribution.
memcached - High-performance, distributed memory object caching system
Oracle TimesTen - TimesTen is an in-memory, relational database management system with persistence and...
MongoDB - MongoDB (from "humongous") is a scalable, high-performance NoSQL database.