I've always found it interesting to have a nose at who my ancestors' neighbours were, while looking up census entries. It's odd to think my ancestors would have known many of these people -- some of them, possibly, very well; and their various occupations also help to give a feel of the neighbourhood that my ancestors were living in.
But sometimes it's gone further: discovered, in the 1901 census, the (fortunately distinctively-named) young lady, working in the draper's shop just a couple of shops away from the home of an ancestor who emigrated to Australia a couple of years later, who then married him in Australia in 1906. Without that entry, I would never have known how they met. The same census also shows the family my grandmother was later born into living next door to the married big brother of my yet-to-be-born grandfather -- did the two know each other as they were growing up, I wonder, when my grandfather perhaps visited his brother?! And the marriage certificate of my great grandfather and his first wife reveals that they (very conveniently) were next-door neighbours, too!
Just wondered if anybody else had made any interesting/useful discoveries amongst their ancestors' neighbours.