Ryzom is a commercial open source mmorpg. This means that all the game assets are free but you pay a subscription to play on the official servers. It is a very unique skill-based game (there are no levels) that lets you develop your character in any way you prefer, form guilds & buy guild houses, craft, explore its beautiful world and socialize. You can play as one of the four Homin races to protect the living world of Atys against the giant insect invasion known as the Kitin. You’ll just have to Google a bit to find the appropriate native client for your distro.