ErisMUD is a MUD/MOO server/engine written in Python and supports many features: rooms, NPCs, items, player skills and stats, ANSI colors, a help system, damage/healing, etc. SQLite is used to store the data, so no files are required for building (unlike LPMUDs). Short and easy internal commands take care of the development and maintenance.
L2J is an alternate game server for Lineage2. The L2 protocol is reimplemented in the server, so you can play with this server without any modifications to the client (except changing the server IP address). The latest USA live client version is always supported, so don't expect this server to work with the PTS/Korean client.
Phaos is a browser-based MMORPG. It features character creation, the ability to purchase/sell/drop potions, weapons and armor, traveling, inter-player chat, the ability to fight in the arena, explorable dungeons, an admin system, the ability to trade with other players, and the ability to fight other players. A download is available for people running Windows who do not have access to or do not want to set up a Web server.
Quantum Star SE is a browser-based space strategy and trading game played across a mapped galaxy against other players. Utilizing mining, trading, research, and production through easy-to-use tools, QS enables both broad micro-management of resources or more "aggregated" methods. Players compete, combat enemy fleets, and manage planets to become the "last man standing". No user downloads are required, and the game is free to host or play.