Ask HN: Is there a good fork of MongoDB? (like MariaDB to MySQL)

Well first off - I wouldn't consider MariaDB a 'good fork'.

They've done the exact things they claimed Oracle would do, and done the very things they said they wouldn't.

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 all open source).

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.