Most of them have been banned. A few people have one or two alts for legitimate reasons, like I do. Often they were testing accounts for staff members, or problems with old accounts, such as a name that won't work in the IRC chat. But you need permission to get one first.
Of the top of my head I could only name a few: myself, Man of Steel, Mes Tarrant, Vex Kitten, A Roxxor, nerve, Truth Bearer. Uh... That's about it :P