This mod adds a single item (bottlecaps) to the game that currently has no other use than a currency on SMP. It was inspired by the Fallout series.
I was playing on a private server with some friends, and we started opening up some shops to sell things. There was no set currency, so we bartered. I came up with the idea of creating a mod that introduced something as a marketable medium and since we all enjoy Fallout (we all collect bottlecaps IRL to prepare for the coming apocalypse), the currency would be bottlecaps.
Currently, the way to get bottlecaps in survival is to kill hostile enemies (have fun with hyper-inflation). I plan to replace that system with some sort of machine that makes the caps out of experience. I also plan to make this single player friendly by implementing Villager trading.