Unity3D Client And C# Server Framework
Botcraft is a cross-platform C++ library to create bots
Open source clone of the Age of Empires II engine
Java Settlers
Massively multiplayer online game server and client framework
SkipBo! server and Qt client
PlaneShift Role Playing Game immersed into a 3D virtual fantasy world
Free, open-source platform for running LEGO® SERIOUS PLAY® remotely