Opensource Created Custom Viewers For Virtual Worlds like SecondLife.
Cooperative multiplayer graphical RPG and adventure game
Odamex is a free, cross-platform, modern client-server Doom engine
0 A.D. is a free, open-source, cross-platform real-time strategy game.
A 2D multiplayer space shooter game
GameServer is a sophisticated, game server designed for online games
Cross-platform real-time strategy game (3D)
Out battleship is back. Destroy everything in the space!
An online multiplayer real time strategy game
Ur-Quan Masters High Definition
Free MMORPG
An open-source, multiplayer fps based on Cube 2 Engine.
A minimalist multiplayer game inspired by Astro Party, for Windows.
Clone of Urban Assault with new Graphics.
Grepolis utility
OpenKore is an automation tool for Ragnarok Online
Simple meta-server for locating other multiplayer servers
MMOG Framework. Starting place to build a game.