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.

Note that as of Firebird 0.9_4, the locksmith back door has
been closed.

For more information on Firebird and InterBase, see:
http://www.ibphoenix.com/
http://www.interbase2000.org/
http://www.interbase.com/

and of course the firebird page itself:
WWW: http://sourceforge.net/projects/firebird/

For more information on the locksmith back door, see:

https://www.kb.cert.org/vuls/id/247371