I remember I had one about some kind of election being close and they gave me a couple of really nice options for my courthouses everywhere. Since a courthouse is in nearly every city, that was a good one to have. I forget what the bonus exactly was though, maybe somebody else that's had it will remember.
Game I just played I got that cover promotion on all melee right before I launched an all out assault on my first victim. It made it a breeze to take out archers, my swordsmen wound up getting CRIII promotions and being useful until Muskets came around.
Having March for your entire set of Gunpowder would definately be a game changer. I've had that inflation one too, that was nice.
I like that one where you can increase an airport's happiness, that's pretty useful in the late game.