http://www.projectdarkstar.com/
Написан на java, GPL2 opensource, их роадмап можно посмотреть здесь http://www.projectdarkstar.com/technology-roadmap.html
По характеристикам не хуже платных серверов:
Zoneless and Shardless
You can have zones and shards if you want to for game design reasons, but they are no longer required for scaling.
Fail-over capable.
When a server fails one of the others takes over, invisibly to the user.
Completely transparently persistent.
Everything in the game world is automatically persistent. This frees the developer from database work and allows for things like true evolving worlds.
Dupe bug and rollback protection.
Dupe bugs are breaks in referential integrity. Darkstar's transactional nature is designed to preserve referential integrity. Rollbacks occur from server failure before data has been persisted. Project Darkstar continuously streams completed transactions out to the long-term storage
Disaster proof.
Because Project Darkstar is based on an integrated (and custom) enterprise level transactional data storage system, the entire back-end can go down suddenly and without warning and still be brought back online later with a sane world state that is historically accurate to within a few seconds of the accident.
Предлагаю в этом топике делиться опытом скрещивания с Unity