I think an excellent Pigouvian tax would be a tax on emails. Many emails involve a negative

externality (I don't really want to receive them) and almost all the ones I really want to get are

worth much more than a penny or so to the sender. So a penny tax (say) on email would probably

generate large amounts of revenue, mitigate an important negative externality, and have minimal

inefficient disincentives. Since email servers are necessarily centralized and networked and all email senders are ipso facto

connected to an ISP who is charging them for access the transactions costs and evasion problems

seem low.