Bannack, Montana is a ghost town located in southwest Montana, which is currently a state park, but was in the past a bustling town and even the territorial capital.
Like most boom towns in the American West, the town was founded based around mining, in this case the panning and sluice mining of gold. Gold was discovered in 1862, and the town was built within a few years. Like most western boomtowns, the next few years were very busy, with many attendant scandals, such as the fact that the sheriff of the town was actually the leader of an outlaw band on the side. The capital was moved to Virginia City, Montana in 1865 because the most easily accessible gold was already tapped out. The town begin a long decline, with the last house being deserted in the 1970s, although it had actually been made a park earlier.
The town is fairly easy to reach and tour for the interested tourist, at least by Montana standards. It can be reached by following a state highway a dozen miles off of Interstate 15, and then following a well-maintained gravel road for a few miles. It is a fairly popular tourist attraction, with a large parking for tourists and with a gift shop. Most of the buildings are kept open, and visitors can walk in and out of them at their leisure. They are kept structurally sound, but the insides are for the most part not maintained: there is much peeling vinyl and warped wood. Exploring the buildings is interesting, although personally I didn't get much of a feeling of ghostliness and abandonment, perhaps because of the fact that I am just one of many tourists looking through the buildings.
There are many other ghost towns in Montana, some much more ruined, but Bannack is a good place to start to see a little bit of the history of the old west.
Sources:
http://www.bannack.org/