Well here are some great singers that are not on the list yet:

James Hetfield (Metallica): During the first 17 years of Metallica, he did a phenomenal job, however in the 15 years after that he was meeeeh, but still the old days earn him a place as a great vocalist

Tommi Joutsen (Amorphis) : well this dude can sing clean and growling and in more than one range in both, and mix both of them and nail them pretty well on stge and off stage, even sing to accoustic music, one really fine vocalist

D. Randy Blythe (Lamb of God) : He has a pretty distinct growl, and can have quite meddle with his growl, and recently he's shown that he can sing clean to a bluesy tune

Nell Sigland (Theater of Tragedy): Awesome chick , and has a pretty amazing voice don't thing there is anything else to add there.

And the list goes on and on, might add some later