>>MySQL and Postgres are free, last I heard.
>
>Interestingly, a new player in the PB-class DB field is based on PostgreSQL: http://www.dbms2.com/2008/08/25/greenplum-is-in-the-big-leagues/

Yeah, and I've been interfacing with Firebird a little bit recently. Lots of options. M$ is still the extreme heavyweight in this arena, however.