Osgiliath:

The first capital city of Gondor, Osgiliath was decimated in the Plague, then the capital was moved to Minas Anor in the year 1640 of the Third Age. Osgiliath was abandoned by the time of the War of the Ring, and the bridge was destroyed in the year 3017-3018 as was told by Boromir at the Council of Elrond.

At it's founding, Osgiliath was the site for the chief of the palantíri that Elendil brought to MIddle-Earth. It was lost during the Kin-strife between 1432 and 1448 of the Third Age.

The Old Forest:

The forest on the eastern borders of Buckland. On it's eastern edge was the Barrow-Downs. It was a frightening place where few hobbits would enter. According to Fangorn, the Old Forest was one of the remaning fragments of the original forests that covered Western Middle-Earth, as was Fangorn Forest. It is recorded in the Red Book that the trees were known to move at times, and were actually threatening to the Hobbits, as Merry, Pippin, Frodo and Sam found on their journey. It is likely that a large number of the trees were Huorns, such as Old Man Willow.

Tom Bombadil and his wife Goldberry lived in the Old Forest on the banks of the Withywindle.

Orodruin:

The volcanic mountain within the land of Mordor, this is the location where Sauron forged the One Ring. The name translates as Mount Doom. As the place where the One Ring was forged, the Cracks of Doom within the mountain are the only place where the Ring could be destroyed.

Orodruin was an active volcano through the Second and Third Ages in the periods when Sauron was a strong force in Middle-Earth and in residence in Mordor. Sauron clearly had some form of control over the activity of the mountain, as it was used as a weapon during the War of the Ring, when the smoke blocked the light from a large part of western Middle-Earth.

Orthanc:

The tower and city within the ring of Isengard. Orthanc was first built by the Men of Gondor. Though the land surrounding was given to the Rohirrim, Isengard was retained by the Stewards of Gondor. In the time of the Steward Beren, the Keys of Orthanc were given to Saruman. At the end of the Third Age of the Sun, Sauruman had changed Orthanc and Isegard to a mockery of Barad-Dûr. Orthanc survived the attack of the Ents in the year 3018 and was given back to the King of the Reunited Kingdom, Aragorn. This was the site of one of the four palantíri until the year 3018 of the Third Age.

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