Popular Relational Databases

There are a number of popular database system that provide relational models. In the old days these were only providing relational databases, but with the popularization of the so-called “NoQSL databases” many of these systems started to include elements that are not relational. In other words, these system can be used as RDBMS-es, but often also as other types of databases.

Propriatery

• Oracle
• Microsoft SQL Server
• IBM DB2
• IBM Informix
• SAP Sybase
• Teradata
• Microsoft Access

Open Source

• MySQL (though it belongs to Oracle)
• PostgreSQL
• SQLite (embedded, public domain)

Wikipedia has long list of relational database management systems.