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.I know my life would improve under such a tax.
Even better, if possible, might be to have the recepient set the price! I would happily raise mine to a dime, and let the government use the revenue to fix the long-term fiscal imbalance or cut other more distortionary taxes.
--Greg Mankiw on a welfare-improving tax