End-to-end encrypted private file sharing darknet that basically appears as a regular folder and doesn't share anything outside your friends. Given some critical mass, that'd just be way too user-friendly for sharing files with $FRIENDS.

Now I'd like to know what the MAFIAA can do about that? This is where it's headed anyway, what the world needs is a mass-market darknet software and this sort of thing just might be it, given Dropbox's popularity.