What is the equivalent of MSSQL to Open Source

David mentioned the main big ones out there which are deployed in a ton of environments. If this is for personal use, on a small project, you may find SQLite more suitable. I wouldn't consider it a fully-fledged replacement, but it works well for personal projects. SQLite is the most widely deployed SQL database engine overall as it is used in software like Skype, Firefox and Solaris 10.