Redis - Redis is an open source in-memory data structure project implementing a distributed, in-memory key-value database with optional durability.
Hazelcast - Clustering and highly scalable data distribution platform for Java
KeyDB - KeyDB is fast NoSQL database with full compatibility for Redis APIs, clients, and modules.
memcached - High-performance, distributed memory object caching system
Dragonfly DB - Dragonfly - Scalable in-memory datastore made simple
MongoDB - MongoDB (from "humongous") is a scalable, high-performance NoSQL database.