The Penny Black Project
A group of researchers at Microsoft think they may have come up with a solution that could, at least, slow down and deter the spammers.

The development has been called the Penny Black project, because it works on the idea that revolutionised the British postage system in the 1830s – that senders of mail should have to pay for it, not whoever is on the receiving end. The payment is not made in the currency of money, but in the memory and the computer power required to work out cryptographic puzzles.