I just found out about the decentralized digital cash system Bitcoin, by way of an LWN article. It looks very interesting, but as I understand it it can’t scale very far in its current form due to the requirement that a log of all transactions be sent to all clients. One interesting point is that publishing the complete list of transactions doesn’t imply complete transparency, since accounts aren’t linked to users. An account is just a public/private key pair, and any user may create however many accounts she likes. The existence of an account isn’t announced to the world, except implicitly when the account is on the sending or receiving end of a transaction. (Since all transactions are published, however, it’s possible to data mine the flow of money, so the actual anonymity is likely to be limited.)