Photon is a multiplayer development framework obtainable as licensed software and is available via download to developers. Photon comes with SDKs for multiple platforms including Flash, Windows, .NET, Unity3D, Android and iPhone amongst others. Although Exit Games is already known for being one of the most recognized multiplayer engine providers for the mobile market, they have continued to expand their technology to support many platforms from PC to Mac and browsers to deliver a well-rounded multiplayer experience.
http://www.exitgames.com/product/photon
Realtime Server: reliable UDP & TCP
Application Server: HTTP
C++ based ofr Ultimate Performance
Extend it easily in C#
Tens of Thousands of Concurrent Users
Host it in the Amazon Cloud
(и под юнити SDK есть)