I was able to get Spiral Knights working in Fedora, first try. It's java, so if you have Sun/Oracle Java installed it runs perfectly. Not a true MMO, it's more of a co-op game, but it has many MMO features.
Another MMO I got working NEAR perfectly is World of Warcraft. OpenGL mode is smooth and the Launcher/Updater works perfectly in Wine now, so WoW is a easy option for those wanting to get their MMO fix.
Not really a MMO, but kind of falls in that catagory, is Second Life. The Linux client is essentially perfect and there are a lot of "traditional" role playing communities to be found.