Crossfire is a multiplayer graphical arcade and adventure game made for the X environment.
It has certain flavours from other games, especially Gauntlet (TM) and Nethack/Moria.
Any number of players can move around in their own window, finding and using items and battle monsters. They can choose to cooperate or compete in the same 'world'.
To play the game you'll need to have access to a local or remote server.