Bottle cap

Bottle caps are the standard currency in Fallout, Fallout 3, Fallout: Brotherhood of Steel, and Fallout: New Vegas.

Overview
Bottle caps are a popular choice of currency in the wasteland, as the technology to manufacture bottle caps and paint the surface has been lost since the Great War, which greatly limits any counterfeiting efforts. Due to use as currency, bottle caps wear out over time and eventually become unusable. For this reason, organizations that recognize or emit caps as currency (eg. Alice McLafferty's Crimson Caravan) periodically create new bottle caps, to keep the value of a cap at a stable level. Of course, to prevent devaluating the cap, they also seek out and either disable or secure for themselves printing presses capable of creating new caps.

Fallout
Bottle caps are backed by The Hub merchants in the Core Region. The reasoning for their support is that the technology to manufacture bottle caps and paint the surface has been lost in the Great War, which would greatly limit any counterfeiting efforts. Secondly, there is a limited number of bottle caps, which would serve to preserve their value to some degree.

Bottle caps are also called Hubbucks, because they originated from the Hub.

Fallout 2
Since regular money has been introduced, bottle caps have been rendered useless and can only be found in Broken Hills.

Fallout 3
Bottle caps make a return as the standard currency of the Capital Wasteland in Fallout 3. In addition to bottle caps, a form of pre-War currency (most likely American dollars) can also be found in the ruins of DC which can then be sold to merchants in return for bottle caps. Bottle caps can also be put to use in weapon form as one of the major components of a bottlecap mine.

Fallout: Brotherhood of Steel
Apart from regular caps, there is a special kind of bottle cap called the Bawls bottle cap, which comes from Bawls. They can be sold for 50 regular caps each.

Fallout: New Vegas
Bottle caps are just one of the many curencies used by different factions (albeit the one most commonly used in the wastes).