The Dúnedain:

The Men of the West, those descended from the survivors of the destruction of Númenor. The Dúnedain of the south founded the kingdom of Gondor, while those of the north founded Arnor. The surviving Dúnedain of the North Line after the death of its last King became the Rangers, who protected places like the Bree-land and The Shire.

The Dead Men of Dunharrow:

Ghosts, fated never to rest until they fulfill an oath they broke to Isildur. They fulfilled that oath in the days of Aragorn, during the War of the Ring in 3019, after which they never disturbed Middle-Earth again. Freed from the curse laid upon them by Isildur, they probably left the Circles of the World as is the fate for all of the Atani.

The oath sworn by those who later became the Dead men of Dunharrow was to aid Isildur in his fight against Sauron. When the time came for them to fight for him, he found that they had allied themselves with Sauron. However, his curse so frightened them that they remained at home, and fought on neither side of the war.

Dwimmerlaik:

The Rohirrim word for any ghosts. These included the Lord of the Nazgûl. For so he was called by Éowyn.

Dalemen:

The Men of Dale, both before its destruction by the dragon Smaug and after it was rebuilt. There was one peculiarity to the Dalemen, they could understand the speech of thrushes. It was by this method that the weakness of Smaug was told to Bard.
The Men of Dale used to have the trick of understanding their language, and used them for messengers to fly to the Men of the Lake and elsewhere. (H.217)

After the rebuilding of Dale, Bard became its first king.

Dunlendings:

The Men of Dunland. They had a long-running hatred of the Rohirrim who had taken most of their land when it was given by Gondor. This hatred flamed up many times in their history. The last recorded time was the Battle of Helm's Deep. In that battle the Dunlendings were allied with Saruman. They had their own language and customs. Their language sounded to most people to be the screaming of birds and beasts.

Durin's Crown:

The Dwarves name for the Plough, also called the Sickle.

Durin's Day:

The first day of the Dwarves New Year. It was on this day that Bilbo and the dwarves in the quest of Erebor gained entrance to the Lonely Mountain.

The Doom of the Noldor:

The prophecy that was spoken by Mandos concerning the fate of those of the Noldor who followed Fëanor when he left Aman. It was also called the Prophecy of the North, and has been considered as much a curse as a prophecy. The words of the prophecy are:

Tears unnumbered ye shall shed; and the Valar will fence Valinor against you, and shut you out, so that not even the echo of your lamentation shall pass over the moutains. On the House of Fëanor the wrath of the Valar lieth from the West unto the uttermost East, and upon all that will follow them it shall be laid also. Their Oath shall drive them, and yet betray them, and ever snatch away the very treasures that they have sworn to pursue. To evil end shall all things turn that they begin well; and by treason of kin unto kin and the fear of treason, shall this come to pass. The Disposessed shall they be forever.
Ye have spilled the blood of your kindred unrighteously and have stained the land of Aman. For blood ye shall render blood, and beyond Aman ye shall dwell in Death's shadow. For though Eru appointed to you to die not in Eä, and no sickness may assail you, yet slain ye may be, and slain ye shall be: by weapon and by torment and grief; and your houseless spirits shall come then to Mandos. There long shall ye abide and yearn for your bodies, and find little pity though all whom ye have slain should entreat for you. And those that endure in Middle-earth and come not to Mandos shall grow weary of the world as with a great burden, and shall wane, and become as shadows of regret before the younger race that cometh after. The Valar have spoken. (S.103-104)

This doom affected even those who, like Galadriel, had no intention of participating in Fëanor's vengeance.

The Lord of the Rings
The Silmarillion
The Unfinished Tales
The Hobbit
The Complete Guide to Middle Earth by Robert Foster