I've always been fascinated with how underground economies work due in part to my addiction to reading any story about prison escapes. I can clearly remember the mastermind doling out packs of cigarettes to keep someone quiet or acquire a critical piece of the escape plan. Indeed cigarettes were the de facto currency for prisoners, who cannot possess real currency, until smoking was banned in 2004. Since inmates can no longer barter with cigarettes they've turned to pouches of mackerel to fill the need for a common currency inside prison walls.

Even though bartering isn't officially allowed inside prison walls, prisoners are expected to dole out a few pouches of mackerel when you get your haircut. The use of fish by prisoners to create their own economy is a reminder that virtually anything can be used as currency. Even outside prison walls we don't need to rely on government minted tender to complete transactions so long as there are willing parties involved. For all I know there's a subset of people in my town who would gladly accept Chuck E. Cheese tokens or Antartican Dollars for my daily needs.
Unfortunately using something like mackerel as a currency has a huge downside in that it isn't valuable to anyone outside of prison, except perhaps as an afternoon snack. This highlights one of the biggest problems with underground economies, often times there is no easy way for the given currency to be exchanged for a more useful or widely accepted form of money. Personally I think this dilemma represents a big opportunity for someone with ingenuity and prison connections to create a mackerel exchange allowing relatives or soon to be prisoners to purchase the "macks" of inmates getting out, much the same way commodities are traded on the floor of the Chicago Mercantile.

If you open up the Leavenworth Mack Exchange don't forget you owe me 10% or a million "macks", whichever's worth more on the outside!