MariaDB - An enhanced, drop-in replacement for MySQL
PostgreSQL - PostgreSQL is a powerful, open source object-relational database system.
Mimer SQL - Mimer SQL is an SQL-based relational database management system from the Swedish company Mimer...
SQLite - SQLite Home Page
Redis - Redis is an open source in-memory data structure project implementing a distributed, in-memory key-value database with optional durability.
MongoDB - MongoDB (from "humongous") is a scalable, high-performance NoSQL database.