Originally released in 2004, the MMORPG (Massive Multiplayer Online Role Playing Game) game Ryzom was opened up to the open source community in 2010. Developers have provided a free Linux client which can be downloaded from the Ryzom developer web site.
The game is set in the distant future on a far away alien planet, which is actually a huge tree. Various humanoid populations try to coexist and survive in a complex fantastic world loaded with science fiction and adventure. The game is considered one of the best MMORPGs.
See the project web site for more info.