Our session this Saturday had been delayed for five weeks giving me way too much time to come up with the specific whys and hows of a group of primitive drow living in a small cave system directly below a lost temple to a dark god. Very little of what I came up with will be at all relevant to the adventure but it's satisfying to see my players' light bulbs turn on sometimes: "Ohhhh Their ancestors escaped from an Illithid colony and they breed troglodytes to sacrifice to the shrine's undead guardian because they think it's stopping the mindflayers from coming back! Cool, now where's that magic sword we're looking for."

They might not even learn about a tenth of what I came up with but it was fun to do all the same. What are some bits of world building you've come up with for small locations or communities that maybe didn't matter at all but you're proud of?