In recent weeks, we have had new neighbours move in on either side of us. Whereas the previous bunch were happy never to go in their gardens other than to install over-size satellite dishes, these guys both set about their lawns and beds with impressive vigour, quickly turning junk yards in to beauty spots. This is a classic example of a positive externality - an action by another person that benefits us (we now look out over nice gardens), without having to pay a penny.

On the other hand, the guy opposite's steadfast refusal to buy any sort of blind for his overlooked toilet window can politely be described as a negative externality.

What are the things that your neighbours do which improve or reduce your quality of life?