MySQL - The world's most popular open source 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
MarkLogic Server - MarkLogic Server is a multi-model database that has both NoSQL and trusted enterprise data management capabilities.
Aerospike - Aerospike is a high-performing NoSQL database supporting high transaction volumes with low latency.