Also ToroDB (https://github.com/torodb/torodb https://github.com/torodb/torodb) implements the MongoDB protocol. But it uses PostgreSQL as a backend and it's op

Also ToroDB (https://github.com/torodb/torodb https://github.com/torodb/torodb) implements the MongoDB protocol. But it uses PostgreSQL as a backend and it's open source ;P (ToroDB developer here)