Wyvern

If you're a fan of massively multiplayer online RPGs, then you should give Wyvern a try. This award-winning game is similar to a text-based MUD, but with the added bonus of a graphical view. One of the unique features of this game is that if you become a Wizard, you can create your own mods and game areas for other people to visit.

Wyvern comes with a Map Editor that allows you to create entirely new areas for your own world. Additionally, the game is fully programmable in Java and Jython, so if you're someone who enjoys coding, you can create your own unique aspects of the game using the Wizard developer tools. You can create weapons, monsters, treasures, quests, guilds, and more.

Currently, Wyvern is in beta test mode, which means that the creators are encouraging players to playtest the game, submit bug reports and feature requests. Before you log in, make sure to read the Official Wyvern Beta Test Rules. To be able to play the game, you'll need Java 1.4 or later.

Overall, if you're looking for a game that lets you unleash your creativity and play around with game mechanics, Wyvern might be worth a shot.