The open-source InterBase(tm) 6.0 spin-off (Classic server version)

InterBase is a world-class DBMS with a history too rich to detail
here.  Once selling for several thousands of dollars per license,
some very big players use it worldwide.  Inprise has open-sourced
InterBase 6.0, and a lively bunch of folks has charged themselves
with seeing that it lives on through the Firebird project.

For more information on Firebird and InterBase, see:

WWW: http://sourceforge.net/projects/firebird/
WWW: http://www.ibphoenix.com/
WWW: http://www.interbase2000.org/
WWW: http://www.interbase.com/
WWW: http://www.firebirdsql.org/