Deliantra is a free software (some people say: open source), cooperative multiplayer RPG and adventure game, much similar in style (and setting) to nethack and moria, but is fully graphical and runs in realtime. It is set in a classical medieval environment with ubiquitious weapons and magic.
What makes this game special is its depth and the sheer amount of maps to explore: The world is vast (multiple continents, over 4000 manually designed maps, hundreds of monsters and countless variations of generated content) and complex (16 unique races, 17 classes, 40 skills and hundreds of spells to choose from and combine with each other), so there is a lot to explore. The gameplay is open and sets very few limits on character development. And of course a lot of fun is to be had by playing cooperatively with others exploring the same world.
• Version 2.04 is a small bugfix and feature enhancement release of the client. A Japanese font is included with the distribution now and a workaround of the incompatible ABI change in pango (font engine) has been implemented. Another bugfix related to connecting to the server went into the windows release.