Who were the Ringwraiths?

[Halbarad]

You forget that Sauron used men too. Let's look at the Mouth of Sauron. Of him it is said that he was a living man and a great sorcerer.

...yet this was no Ringwraith, but a living man. The Lieutenant of the Tower of Barad Dur he was....and because of his cunning he grew ever higher in the Lord's favour; and he learned great sorcery; and he knew much of the mind of Sauron; and he was more cruel than any orc.

A rumoured Black Numenorean he was. This proves that Sauron had powerful Men in his employ: long lived, very powerful, and deeply involved in the big boss' plans. Gothmog could have easily been a twisted man.

I agree that Sauron wouldn't want someone too powerful wresting the ring from Aragorn, if he did indeed have it, so my point falls a little flat. My only counter to that would be that some of Sauron's closest and most trusted servants were there of their own free will, for example the Lieutenant of Barad Dur. Most of Sauron's servants were in fact slaves- orcs, trolls, variags- or men who worshipped Sauron as a God (Easterlings, for example). So, if they were to get the Ring, they would have taken it to Sauron, because the rewards would have been tremendous. Remember, the Ring worked to get back to Sauron, so it wouldn't need to twist a servant taking it back to Sauron. Also, a servant of Sauron would know the fate in store for someone who tried ot keep it away from him, so fear would be a very powerful influence on their actions.

As for the Nazgul not craving power/domain, I said that it was a more contentious idea so your objection is noted. However, I think it is possible because of the arrogance the Witch King shows on both occassions when he speaks. He had cunning and he had a mind that could doubt (read the encounter between him and Eowyn). Look also in The Siege of Gondor, where he says

Old fool, this is MY hour

(emphasis mine). Not "Sauron's hour", but "My hour". Sure the Nazgul were completely in thrall to Sauron, but they had full cognitive power (who would lay a siege without the ability to think laterally or whatever), and the rings were originally given to them because they craved power, so that, to me, says that they had the capacity to be jealous. It's a side issue, though.