Primecoin (http://primecoin.io/ http://primecoin.io/) has been out for a long time. It's a decentralised cryptocurrency which uses prime number chains as its pr

Primecoin (http://primecoin.io/ http://primecoin.io/) has been out for a long time. It's a decentralised cryptocurrency which uses prime number chains as its proof-of-work. I don't think it revolutionised anything, but some mathematicians were a little excited when it was released.