For travellers and people who move around a lot: a personal GPS beacon; software that produces a bayesian prediction of my current/next location; a mail system that delivers mail to the nearest post office.
The system then sends a text/SMS message telling me where I can pick up my mail, or I can request a "stalker" who will bring it to me.
Of course, one would need some way for the stalker to identify you as the person expecting mail. And that's where the hat comes in: a cap with a little red flag, like a rural mailbox!-- sofacrat, May 16 2006random, halfbakery