Regnum Online is a free fantasy massively multiplayer online role-playing game (MMORPG) game (although premium content is available for enhanced game play) which runs on both Windows and Linux and I must say it looks really good.

One thing which makes Regnum Online better than a lot of others MMORPG games (besides it's graphics and gameplay) is it's user base, currently aprox. 540,397 users being registered to Regnum Online.