Open-Source Databases

I am using WordPress as the CMS for my website.  WordPress uses a combination of PHP and MySQL, but here are a variety of other options for open-source databases. 

MariaDB,
http://mariadb.org/

Cassandra,
http://cassandra.apache.org/

PostgreSQL,
http://www.postgresql.org/

Reddit
https://github.com/reddit/reddit/wiki/Install-guide