I've been enjoying reading "So You Want To Be A Gamemaster" for the last month or so. Even though his discussion of Rumor Tables focused on hexcrawls, I am excited for its potential in my new city-based proactive game.
The basic concept for hexcrawls is that the rumor table is populated with nuggets of information PCs can pick up that will direct them to hexes (get the story moving). Characters might pick them up chatting with a merchant or tavern keep, as 'treasure' in a foe's things, a mysterious note passed to them, as the result of some divination magic, or just shared between sessions as stuff characters heard going around town.
Fundamentally, the Rumor Table is a mechanism to get enough world information to players that they can make meaningful (informed) choices. And it solves a problem I often struggle with and was anxious about for the proactive game. In my city campaign they will be a mechanism to share faction goals, progress on those goals, and introduce NPCs they might look for to connect with those goals. I can probably work in some city crawl type clues as well, particularly if getting to the location can start them on one of their goals.