Also beginning today is Cambridge's Park & Read scheme. Cambridge residents are strongly encouraged (by me, I hasten to add, not the Council) to add value to the scheme by leaving a more eclectic selection of books on the buses, or indeed anywhere else that takes their fancy.
I left a copy of Alain De Botton's The Art of Travel on the train to Zermatt once, but that was by accident. I'd leave more books on trains if I didn't have a sneaking suspicion that they'd just be thrown away at the end of the day. Perhaps I should leave books in doctors' waiting rooms, where (as rysmiel and friends observe) they clearly never throw anything away.
Yes, I have seen the Bookcrossing project.