After reading this answer to my question about wraiths, ghosts, and wights in Middle-earth, I understand that Ringwraiths are the result of people who have been corrupted by using the rings, but that there is other dark magic that can corrupt and turn one into a wraith.
Are there other wraiths in Middle-earth? And what was it that corrupted them and turned them into wraiths?
Then Beren was aware in his dream of a form that came to him across the water, and it was a wraith of Gorlim; and it spoke to him declaring his treachery and death...
(src: The Silmarillion)
What corrupted him? He was killed by Sauron after betraying his company.
Yet men said that the horn was still heard at times in the Deep and the wraith of Helm [Hammerhand, of Helm's Deep - DVK] would walk among the foes of Rohan and kill men with fear.
(src: LOTR, Appendix A)
Corruption source: Unknown
It is speculated that those hurt by Morgul-blades (wielded by Nazgûl) like Frodo would possibly become wraiths in thrall to Nazgûl.
It almost happened to Frodo:
Pippin declared that Frodo was looking twice the hobbit that he had been.
“Very odd,” said Frodo, tightening his belt, “considering that there is actually a good deal less of me. I hope the thinning process will not go on indefinitely, or I shall become a wraith.”
“Do not speak of such things!” said Strider quickly, and with surprising earnestness.
and when healing in the House of Elrond from the Morgul blade wound:
“They tried to pierce your heart with a Morgul knife which remains in the wound. If they had succeeded, you would have become like they are, only weaker and under their command. You would have became a wraith under the dominion of the Dark Lord; and he would have tormented you for trying to keep his Ring, if any greater torment were possible than being robbed of it and seeing it on his hand.”
Tolkien used the term barrow-wight for wraiths who inhabit and defend tombs. Also, the oathbreakers, Aragorn's army, are much like wraiths as well. Each of these character types are trapped between life and the after life.
Existence of Barrow-wights is connected with necromancy; from "Morgoth's Ring" of the History of Middle-earth series:
But it would seem that in these after-days more and more of the Elves, be they of the Eldalië in origin or be they of other kinds, who linger in Middle-earth now refuse the summons of Mandos, and wander houseless in the world, unwilling to leave it and unable to inhabit it, haunting trees or springs or hidden places that once they knew. Not all of these are kindly or unstained by the Shadow. Indeed the refusal of the summons (of Námo to Mandos) is in itself a sign of taint.
It is therefore a foolish and perilous thing, besides being a wrong deed forbidden justly by the appointed Rulers of Arda, if the Living seek to commune with the Unbodied, though the houseless may desire it, especially the most unworthy among them. For the Unbodied, wandering in the world, are those who at the least have refused the door of life and remain in regret and self-pity. Some are filled with bitterness, grievance, and envy. Some were enslaved by the Dark Lord and do his work still, though he himself is gone. They will not speak truth or wisdom. To call on them is folly. To attempt to master them and to make them servants of one’s own will is wickedness. Such practices are of Morgoth; and the necromancers are of the host of Sauron his servant.
Some say that the Houseless desire bodies, though they are not willing to seek them lawfully by submission to the judgement of Mandos. The wicked among them will take bodies, if they can, unlawfully. The peril of communing with them is, therefore, not only the peril of being deluded by fantasies or lies: there is peril also of destruction. For one of the hungry Houseless, if it is admitted to the friendship of the Living, may seek to eject the fëa from its body; and in the contest for mastery the body may be gravely injured, even if it be not wrested from its rightful habitant. Or the Houseless may plead for shelter, and if it is admitted, then it will seek to enslave its host and use both his will and his body for its own purposes. It is said that Sauron did these things, and taught his followers how to achieve them.
Such elvish wights could be quite large in numbers (Avari elves were numerous, many could fall under Dark Power of Morgoth or Sauron). And of course there were always those who practiced sorcery (Black Númenóreans, other peoples), many of them. "Host of Sauron" implies large numbers.
I am not sure if Lord of the Rings Online is considered for canon, but there are many wraiths besides the 9 Ringwraiths in that narrative. Wraiths are able to convert those they sap the life from into wraiths themselves, so from this I would assume many wraiths have existed in Middle-earth.
In addition to the Nazgûl, the Cargûl (Sindarin for "Red Wraith") are lesser Wraiths enslaved by the Nazgûl. Most are likely victims of the Nazgûls' Morgul-blades, a terrifying supernatural weapon that saps a person's life. Those wounded by a Morgul-blade will find themselves fading from the mortal world and drawn into the Wraith-world, a spiritual plane just beyond our perception of reality. Once the transition is complete, the person is transformed into a Wraith and, like the Nazgûl themselves, can only be seen by mortal eyes when enshrouded in dark robes. blurb source Again, they may not be canon.
