Redis - Redis is an open source in-memory data structure project implementing a distributed, in-memory key-value database with optional durability.
MySQL - The world's most popular open source database
memcached - High-performance, distributed memory object caching system
PostgreSQL - PostgreSQL is a powerful, open source object-relational database system.
MongoDB - MongoDB (from "humongous") is a scalable, high-performance NoSQL database.
SQLite - SQLite Home Page