(was "Database used by wikidata")
We would LOVE for more developers or systems administrators to help support MediaWiki core on other RDBMSes; one of the most helpful things you can do is help look at the changes other developers are making that might affect your preferred RDBMS, and provide testing and code review.
https://www.mediawiki.org/wiki/Code_review_guide
For instance, here's a search for not-yet-merged commits that mention PostgreSQL in their commit summaries:
https://gerrit.wikimedia.org/r/#/q/message:postg,n,z
Anyone can comment on those code changes in Gerrit by getting "developer access" which you can get instantly. See https://www.mediawiki.org/wiki/Developer_access .
Please see https://www.mediawiki.org/wiki/Database_testing for lists of things to test, and feel free to add to "Creating a test plan for databases".
As Aude pointed out, there is at least one db in the WMF cluster now running MariaDB. More details: http://lists.wikimedia.org/pipermail/wikitech-l/2012-December/064994.html