PostgreSQL - PostgreSQL is a powerful, open source object-relational database system.
MySQL - The world's most popular open source database
SQLite - SQLite Home Page
Microsoft SQL - Microsoft SQL is a best in class relational database management software that facilitates the database server to provide you a primary function to store and retrieve data.
MariaDB - An enhanced, drop-in replacement for MySQL
MongoDB - MongoDB (from "humongous") is a scalable, high-performance NoSQL database.