PostgreSQL - PostgreSQL is a powerful, open source object-relational database system.
Amazon RDS - Easy to manage relational databases optimized for total cost of ownership.
Database Labs - Database Labs offers Postgres as a service.
MySQL - The world's most popular open source database
SQL Azure - Microsoft Azure Cloud SQL Database is the developer’s cloud database service. The Azure database as a service is your solution to building and monitoring applications quickly and efficiently.
MariaDB - An enhanced, drop-in replacement for MySQL