Wraiths

Wraiths in Lord of the Rings were like half-spirits, or someone who has been killed yet lives on by some dark sorcery (necromancy or a Morgul-blade), or someone who died in darkness or serious dishonor.

First Age
During the First Age, the love sick Gorlim having betrayed his comrades in arms appeared to Beren as a Wraith, confessing what he had done.

Second Age
During Sauron's influence and rule Middle-earth, he swayed and corrupted nine powerful Men of Númenórean and Easterling stock giving them each Rings of Power. Afterwards, they slowly faded into darkness becoming Ringwraiths enslaved to the will of Sauron to roam the world until their end.

Third Age
The Dunlendings feared that Helm Hammerhand was a Wraith that wandered through Rohan as its protector.

No other recorded evidence of any other Wraiths are recorded as existing in Middle-earth, save for the the Ringwraiths, the Uliari, who continued in the East and in Mordor and when the Ring was discovered came out to search for it. When Frodo is stabbed by the Witch-King of Angmar in the first book he is infected by the evil magic from a Morgul blade; if Glorfindel had not saved him he would have eventually become a Wraith himself.