In January, the online discount store Overstock.com—which sells everything from home goods to electronics to furniture—made news when it became the first major retailer in the world to accept Bitcoin. First introduced in 2009, Bitcoin is the oldest and, by far, the best known of the more than 40 digital currencies that have sprouted up in the past few years. But until recently, the money, which is linked to no one nation and is not regulated by a central bank, was used mainly by collectors to purchase and sell goods and services among themselves.