Firebird is a free relational database that allows many SQL features and has existed since 1981. It supports indexing, primary key, foreign key, stored procedures, and many more. You can use it in many platforms of Linux and Windows. I liked using it because it's free. The only problem is with its IDE. It is really hard to find a user-friendly IDE that is also free. Although there is Flamerobin, I didn't like using it. The commercial ones like EMS for Firebird and IBExpert. I just wish the open source community also comes up with a comprehensive IDE for Firebird. It also took a very long while for Firebird to come up with another version. Maybe because it is almost complete.