I'll side with Alec and littledarwin, old, old Shimano. If it's removal for a good clean then it's fairly straighforward if you have two chain whips. if it's removal to replace cassette you're SOL, they're very hard to find these days.
As already suggested Sheldon Brown has some suff about upgrade possibilities, but that's not always an option either. There's a version of dirt-cheap AND old-old Shimano where the body isn't replaceable by any means short of some serious lathe work. If you've got one of those its time to consider replacing the hub.