Photon Engine - Independent networking engine and multiplayer platform.
LTSP - The Linux Terminal Server Project adds thin-client support to Linux servers.
Unity Multiplayer - Create real-time, networked games.
Thinstation - Thinstation is a basic and small, yet very powerful, open source thin client operating system...
SmartFoxServer - Massive multiplayer platform
Nakama - Nakama is an open-source distributed social and realtime server for games and apps.