Borland's RDBMS, currently at version 6.5, which runs on Linux, Windows and Solaris.
Although Borland did release version 6.0 as open-source (and it is still available from SourceForge), they have not released any fixes or code after that, and the latest features are only available in their commercial version. The Firebird project have forked the code and are continuing open-source development.
It is heavily multithreaded, provides "event alerters" (a way for client applications to "hook" into the database and be notified when certain things happen), has unicode support, provides ODBC and JDBC interfaces, is well-integrated with Java and has standard features such as stored procedures, triggers and UDFs (User Defined Functions).
The main advantage of Interbase is probably it's close integration with Borland's development products, such as Delphi, Kylix, C++ Builder and JBuilder, as they can directly communicate with Interbase rather than having to go through the BDE or other type of interface.