Dol Guldur - This tower in Mirkwood Forest became Sauron's abode after Barad-dûr was destroyed at the end of the Second Age.  It was known as the "Hill of Sorcery" and Sauron was often referred to as the Necromancer.  While he regained his former power, his minions rebuilt his dark tower in Mordor.

"Now the Shadow grew ever greater, and the hearts of Elrond and Mithrander darkened.  Therefore on a time Mithrander at great peril went again to Dol Guldur and the pits of the Sorcerer, and he discovered the truth of his fears, and escaped.  And returning to Elrond he said: 'True, alas, is our guess.  This is not one of the Úlairi, as many have supposed.  It is Sauron himself who has taken shape again and now grows apace..." ~The Silmarillion (Of the Rings of Power and the Third Age)