Alt.prose(and perhaps now rec.arts.prose, for all I know)has had a periodic "Three Word Story" assignment for a while, but I've discovered that four words, especially if one or more is unusual, provide a strong enough framework to build a story around. (Though if they're too unusual, it becomes a chore to try to incorporate them all.)
The following four-word stories are available here:
The Den of Ubiquity / Aaron V. Humphrey / alfvaen@gmail.com