MongoDB: Stashing unstructured JSON data that you don't really know how you might want to query later.

Also, getting up and running with an investor demo ASAP with zero technical fuss, because you have a startup idea but you're broke and can't pay your next month's rent unless you either (A) finish this demo and get that investor money next week, or (B) quit working on your idea and take the Google offer. (Yes, I've actually been there.)