PostgreSQL

PostgreSQL is a powerful, open source object-relational database system. It has more than 20 years of active development and a proven architecture that has earned it a strong reputation for reliability, data integrity, and correctness. Its SQL implementation strongly conforms to the ANSI-SQL:2008 standard. It also has a host of extensions and advanced features. Best of all, PostgreSQL's source code is available under a liberal open source license: the PostgreSQL License.