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Global is the global server www.muonline.com. It is free and has two server chains wigle and maya. When you create a char, you create it on either server chain, and can play it on all the subservers in that chain (wigle1 wigle2 etc). Global has in my opinion, by far the richest community of any mmorpgs I have played (EVE, WOW, DaoC, AC, Ragnarok +many more), for better or worse. When you play, its not just you against mobs, its you against scammers, hackers, ksers, pks etc. (people have even pretended to be girls on msn to cheat me of my account ^_^)
Then there is muphilipines. Its in english and a pay-to-play server with better support and newer version. www.mobius.ph. Earlier it also had wigle and maya serverchains, but has merged those into a chain called Titan. I think it also has a serverchain called Bahr.
Private servers. I dont know how legal these are, but they usually have high exp rate and small communities
Then there is the rest of the non english localized official servers like muthai, muvn etc. Global has blocked ips from those countries so they can only play they local ones.