During the time period of The Hobbit, a character is mentioned called the Necromancer. Radagast even (in the movie) specifically states that the Necromancer can "summon the spirits of the dead." (It's been a while since I read the book so I don't know if this exact phrasing is used there, but given the name it seems reasonable.) This character is later revealed to be Sauron.

Since when can Sauron summon the spirits of the dead? He can extend life ad infinitum, like Gollum's, for example, and he can twist people into half-alive monsters, like the Ringwraiths, and he himself seems impossible to kill so long as the One Ring remains, but I can't recall ever actually seeing him "summon" something that was once fully dead. In fact, the only character I remember doing that was actually Aragorn, though he merely commanded them and didn't raise them himself.

Is this just an unfounded rumor about the power in Dol Guldur, some hushed-whisper nightmare that isn't actually an accurate description, or is there a reason he is called the Necromancer?

It seems like if Sauron could actually resurrect the dead, what little chance Middle Earth had of defeating his hordes would be long, long gone...