OpenMW is an attempt to reimplement the popular role playing game Morrowind. It aims to be a fully playable implementation of the game that will run on Linux, Windows, and Mac OS X. No game data is distributed with the code; the user must already own a copy of Morrowind to use the software.
Vendetta Online is a multi-platform 3D space-combat MMO for Windows, Mac, Linux/32 and Linux/64. The game permits players to interact in a vast online galaxy; fighting, trading, and mining their way to success. A "twitch" type real-time combat model makes true player skill the deciding factor instead of character advancement. Massive capital-ship battles shake the foundations of the major nations at war, while others struggle to push back the encroachment of the AI "Hives". A no-strings free trial is available.
Choria is a game set in the land of chores. It is a 2D MMORPG in 800x600 with 3D accelerated graphics. Its features include doing chores, slaying monsters, killing goblins, getting great loot, earning money for college, and vacuuming virtual carpet. You can play with builds that include up to 30 different skills. PVP play is open and always available.
EZDM (ezdungeonmaster) is a simple set of tools for dungeon managers, targeting AD&D 2nd edition rules. It's not an RPG game, but some helper tools for people DM-ing real games, and is designed not to get in the way of roleplay and storytelling, but simply help do the math during encounters and leveling. Dice rolling can be done by the computer, or the tools can be set for manual dice, in which case you simply enter the results of the roll. All game data, charts, and character sheets are stored in JSON format. The EZDM kit defaults to using a simple dialog-based GUI which requires easygui, but a console interface is also available.
Vallheru is a turn-based, fantasy MMORPG written in PHP and MySQL (but may work with many other databases) with Smarty Template Engine, ADOdb, and PHP Mailer. It is based on Gamers-Fusion 2.5 with many modifications in security, speed, and features. It is currently available only in Polish.
Arx Libertatis is a cross-platform, open source port of Arx Fatalis, a 2002 first-person role-playing game developed by Arkane Studios. Arx Fatalis features crafting, melee and ranged combat, and a unique casting system where the player draws runes in real time to effect the desired spell. The Arx Libertatis source code is based on the publicly released Arx Fatalis sources. This does however not include the game data, so you need to obtain a copy of the original Arx Fatalis or its demo.