Bottle cap

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

Background
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.

Their relative scarcity made them a readily available currency for Hub merchants in the 22nd century, although by mid-23rd century, they were replaced by NCR-backed golden dollars. However, in an ironic twist of fate, caps once again became a regularly-used currency when the destruction of NCR's gold reserves by the Brotherhood caused the value of a dollar to fall dramatically.

For similiar reasons, the East Coast merchants also recognize bottle caps as a currency, although who backs them and guarantees their value is unknown.

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 by discovering Typhon's "treasure" in a well with the help of the treasure hunter Micky. The Chosen One will uncover a bag of 10,000 caps, however they are completely worthless during this era.

Fallout 3
Bottle caps make a return as the standard currency of the Capital Wasteland in Fallout 3. In addition to standard methods of acquiring bottle caps (finding, trading, stealing, or even killing) the player will gain a single bottle cap for each bottle of Nuka-Cola they consume. Bottle caps can also be put to use in weapon form as one of the major components of a bottlecap mine.

Fallout: New Vegas
Bottle caps are just one of the many currencies used in the Mojave Wasteland, albeit the most commonly used, as well as the "base" form of currency used when dealing with merchants. In addition to Nuka-Cola, bottle caps can also be acquired from drinking Sunset Sarsaparilla. Bottle caps can be found in both Nuka-Cola and Sunset Sarsaparilla variants, the latter of which appears different in-world and internally is considered a separate item; however, picking up a Sunset Sarsaparilla cap simply adds to the player's total bottle cap count.

Two additional variants of bottle caps exist within Fallout: New Vegas:
 * Sunset Sarsaparilla star bottle caps are a rare variant of the Sunset Sarsaparilla bottle cap, featuring a blue star on the underside of the cap. These caps have a 1-in-20 (5%) chance of being found when drinking a bottle of Sunset Sarsaparilla, and otherwise can be found in various locations throughout the Mojave Wasteland.  These caps are part of the quest The Legend of the Star and thus cannot be sold.  Once the quest is completed picking up a Sunset Sarsaparilla star bottle cap will simply add to the player's total bottle cap count.
 * Counterfeit bottle caps can be found inside the Cap Counterfeiting Shack. These "fake caps" appear identical to standard Nuka-Cola bottle caps, but have no trade value and cannot be used for any monetary purpose.  Only 18 exist within the game, all found inside the Cap Counterfeiting Shack.

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.