It's been a while since I've mentioned it, but it's come a long ways and I just released a new version.
Game machine is an open source server platform that excels at large virtual worlds with lots of players in the same area. It's focus is on solving a core set of hard problems not just networking. It has built in persistence that scales well. The best space optimization of any platform I'm aware of, and a well defined structure for writing game logic that runs concurrently. Plus a bunch of built in functionality such as multiple characters, groups/chat, area of interest, etc..
The default client is for Unity. Biggest market it's where I put my focus for all the client side implementation. But it's fairly simple to integrate with the server as the core protocol is protocol buffers, and I moved stuff like reliable messaging to a higher level of abstraction. So the networking layer is very dumb (as it should be IMO).
I wrote a completely functional open world mass pvp combat mmo in under 3 months using it, it was an eat your own dogfood project. And all of the server side code for that is also open source and comes with. So there are a ton of examples for how to solve most of the things you might tackle in an mmo.