One of my favorite things about the original Forgotten Realms setting for D&D (the grey box) was the list of rumors that it included. They were broken down by month, and there were a couple of years worth of them. Each rumor was a little adventure hook, and sometimes they played on the assumed course of events from earlier rumors.
As a GM, this gave me a ready source of adventure ideas, and I got some great mileage out of it. And although this never came up back then, if I’d had players who liked to ask around in every town about what was going on in the region, those rumors would have made great unplanned answers to their questions.
This is an extension of constantly jotting down your ideas (you should never be far from a pad!). Whenever you think of a nifty rumor, write it down. You can use them to advance side plots, to answer surprise questions from your players, or to provide fuel for adventures down the road — and this list will work equally well regardless or system or genre.