Microsoft

http://en.wikipedia.org/wiki/Microsoft_SQL_Server, often shortened to just SQL server.

Microsoft. Closed source.

T-SQL http://en.wikipedia.org/wiki/Transact-SQL is the name of the primary language for the SQL server.

Only runs on Windows: http://stackoverflow.com/questions/12470335/how-do-i-install-microsoft-sql-server-for-linux-fedora

Default port: 1433.

Has a freemium limited “Express” version. It limits table size (10Gb range in 2015), processor and RAM usage: http://www.microsoft.com/en-us/server-cloud/products/sql-server-editions/sql-server-express.aspx