The engine of the mmorpg Ryzom was released as open source:
"Ryzom Core is the open-source project related to Ryzom Game. Written in C++, Ryzom Core contains the whole code (client, server, tools) used to make the commercial MMORPG Ryzom. Ryzom Core is a toolkit for the development of massively multiplayer online universes. It provides the base technologies and a set of development methodologies for the development of both client and server code."
"Ryzom Core is the open-source project related to Ryzom Game. Written in C++, Ryzom Core contains the whole code (client, server, tools) used to make the commercial MMORPG Ryzom. Ryzom Core is a toolkit for the development of massively multiplayer online universes. It provides the base technologies and a set of development methodologies for the development of both client and server code."
Repository: https://bitbucket.org/ryzom/ryzomcore/overview
Confluence: https://ryzomcore.atlassian.net/wiki/display/RC/Ryzom+Core+H...
http://ryzomcore.org/