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Everquest - 2000 - '02Anarchy Online - '01-'02Earth and Beyond - '02-'04Star Wars Galaxies - '03-'06('07)World of Warcraft - '04-'07Age of Conan - '08 - shelved.-Waiting on-Star Trek OnlineSW: The Old Republic
Linux is actually a fantastic gaming platform. The problem is convincing developers to code for it. Also (strictly my opinion) the very vocal loudmouths who proclaim "if it isn't open source software, I'll never install it" and other such garbage drives away the developers as well. OSS and other forms of Free Software are great, but the bottom line is professional game developers are in business to make money (hence the "professional" title) and they're not going to open source their code, at least not until it's run its course, like id does with its older Quake engines.
However, if you have a good Linux distro (there are so many it's not even funny, I prefer PCLinuxOS myself, but a lot of people like Ubuntu, Mandriva, or SUSE) and the linux drivers installed for your video card, the two "big names" in getting your Windows games working are WINE which is totally free and open source, and Cedega which was originally based on WINE but has long since gone its own way. Cedega is closed source and requires a payment, something like $5/month in 3 month packages (basically, if you don't subscribe, it's $15 and you get 3 months of official support and the ability to vote for new features) if I recall correctly.
There are also a couple other alternatives, but in general Cedega and WINE has the most support and get the most games playing well.
I can't recall the names right now, but I did come across a few smaller MMOs that have native Linux clients too.