Well first off - I wouldn't consider MariaDB a 'good fork'.<p>They've done the exact things they claimed Oracle would do, and done the very things they said they wouldn't.<p>I'd consider Percona Server or Percona XtraDB Cluster to be better versions of MySQL (unlike MariaDB, they keep compatibility with upstream, and also unlike MariaDB their software is <i>all</i> open source).<p>Percona also happen to maintain a fork of MongoDB (Percona Server for MongoDB). I can't say I've used it because honestly after one client insisted on using Mongo I've never wanted to even consider using it again.