Superline Gamemaster's Series: A Fistful of Plot Devices, #1

From Introduction:

Superheroes spend a lot of their active time responding to crises: something goes very wrong, and they must save the day. This is a collection of ways to create a crisis, including people, places, and things that can go wrong in interesting ways. They include some (real or impending) catastrophes, and also some basically positive developments that have nightmarish potential in the wrong hands. They’re each intended to provide reasons for lots of different kinds of heroes to feel like responding, suitable for use in many different campaigns. They aren’t complete adventures, but open-ended starting points.