Following one of the links in this thread, I found this editorial which describes the lack of a mission-critical open-source DB.

http://freshmeat.net/articles/view/426/

So what are the OSS options with regard to databases?

Well, as mentioned above, it all depends, to a certain extent, on what one believes OSS is. Still, here are, really, the range of options (and the platforms they run on):

MS Access
With either MS's FrontPage or Macromedia ColdFusion, a winning, low-cost, non-scalable solution. NT only. On every desktop, serious scalability issues, good SQL92 support.

MS SQL Server
An excellent product, one of Microsoft's best (second only to the Visual Studio toolset) that has a number of significant flaws and "issues", but which is nevertheless widely deployed despite its relatively (vs. OSS) high cost. Each release gets better. Version 7 was a quantum leap over version 6.5, and version 2000 is better still, although there are things that I file as "these improvements are like Access", which is not a good thing. NT only, and MS, in it's infinite stupidity/arrogance, has made the licensing agreements for this server more restrictive/costly than in the past. This could help or hurt MS; history will judge.

MySQL
The first real relational database on *nix that was embraced by the OSS community. It has serious limitations but a cross-platform environment and a wide install base of OSS supporters running Linux/Apache/PHP|Perl/MySQL. Like Access, it's everywhere (but on an OSS user's system).

mSQL
Inconsequential.

PostgreSQL
A product of Berkeley, a very Oracle-like database competing with MySQL and (to a lesser degree) Oracle. MySQL has a much larger install base and came earlier. Sad.

Oracle (pick the version)
The "best" (if money and personnel are no object) database out there. It certainly has more features/options than virtually any other database; that is what you pay for. Certain "lite" (limited functionality) versions are available for low/no cost for varying platforms. It requires considerable expertise to operate effectively.