You can make it - Open Source MMORPG Platform
Open Source MMORPG Framework
Complete Open Source and Modular solution for MMO
An open source implementation of the Ultima Online Classic Client
LSJ-Server-Terra (Java Multipurpose Sever connected with MariaDB)
A multiplatform medieval 3D MMORPG
A suite of tools to make running a AzerothCore server on Linux easier
An open-source Phantasy Star Online server for *nix
Source of Mana is a unique 2D MMORPG
Arianne is an engine to develop multiplayer online games like Stendhal
GameServer is a sophisticated, game server designed for online games
An unmodified version of the MMORPG Impressive Title by KovuLKD
Makes running a PSO server on Linux easier
Free MMORPG
An OpenGL client-server game
GameWhizzards is a user created, Open Source 3D MMORPG
Find the best oldschool server with the lowest ping [OSRS Live Ping]
Collection of Tools for Perfect World MMORPG
stro is an open-source cross-platform MMORPG server.
Dark Age of Camelot server emulator
World of Warcraft character converter, supporting multiple emulations