Babelfish interprets the Microsoft SQL Server wire protocol so applications written against that licensed Microsoft software instead can target the open-source and free-of-charge PostgreSQL database server. Babelfish supports the Tabular Data Stream (TDS) and also allows PostgreSQL to understand T-SQL.
While AWS is comfortable with where Babelfish is at for its open-sourcing today with good coverage of common Microsoft SQL Server functionality, they do acknowledge there isn't yet 100% coverage and there may be subtle differences in some areas.
Those interested can learn more about this open-source effort via the Amazon Web Services blog with today's post entitled Goodbye Microsoft SQL Server, Hello Babelfish as well as on the new open-source project site at BaelfishPG.org .