Links to Open source databases

http://blog.capterra.com/free-database-software/
  https://www.postgresql.org/about/
    https://firebirdsql.org
  http://www.itworld.com/article/2827484/enterprise-software/six-free-open-source-databases-with-commercial-quality-features.html
https://mariadb.com
  http://www.cubrid.org/