Lately i've been playing a massively multiplayer game called ATITD (a Tale in the Desert). There are both Linux and Windows clients available (use window OpenGL mode in linux). You start out as a nomad with nothing, you must adventure and learn how to build up your empire. Though the game features no killing, the groups and guild aspects are a lot fun. If you're not sure if you'd like it or not and don't feel like paying the monthly charge, the first 24 game hours are a free trial period.