Have you ever stayed in a hostel with a shelf (or more likely, a bookcase) of travel/leisure reading just there for the taking? The idea being, if you finish your book while you're in residence, leave it behind and select something new, whether it be a Rough Guide to your next destination, some light YA fiction, or maybe even a chunky biography of some now-deceased heavyweight politician or diplomat.
Personally, I have trouble leaving my own book behind in exchange for something different, and yet I can appreciate the trade-off. Why carry around something you've already finished when you could lighten the load, or at least maintain it, by leaving your book for someone else to enjoy, and taking along an interesting looking cast-off?
What's the best trade you've made while traveling? Or do you find it impossible to leave one of your own books behind?