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.
PostgreSQL - PostgreSQL is a powerful, open source object-relational database system.
Oracle DBaaS - See how Oracle Database 12c enables businesses to plug into the cloud and power the real-time enterprise.
SQLite - SQLite Home Page
MongoDB - MongoDB (from "humongous") is a scalable, high-performance NoSQL database.