Heroku Postgres - Heroku Postgres is an SQL database as a service.
PostgreSQL - PostgreSQL is a powerful, open source object-relational database system.
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.
MySQL - The world's most popular open source database
Amazon RDS - Easy to manage relational databases optimized for total cost of ownership.
SQLite - SQLite Home Page