Grindewald. He was a dark wizard, Dumbledore defeated him.
Herpo the Fool, also dark, but I don't know much about him.
Salazar Slytherin is some times considered dark, but it depends of the interpretation of history.
And all of the Dead Eaters followers of Voldemort falls into the cathegory of dark wizard, I believe: Nott, Malfoy, Lestrange, Jugson, Dolohov, Rookwood, Crabbe, MacNair, Avery, Mulciber... and so on.

And Voldemortist... our today's Voldemort probably, and his follower.