I do a lot of collaborative writing; I currently have three on the go, though one I've taken over most of the writing portion while my collaborator gives me ideas. They all tend to be post-apocalyptic science fiction, as is a fair amount of what we read and pass around to each other. But the focus isn't on the end of the world, it's on what happens after, in the days-weeks-months after everything changes. The way it ends isn't usually important, either; the most recent collaboration the end of the world was decided based on an article I'd read in the paper that morning, with no real emotional investment in it or plan to explore how we got to that point in the story.
A concept bandied about in science circles nearly as much as science fiction is that of a singularity, an idea or instant or tipping point beyond which the future is unrecognizable and can't be accurately predicted. With the rapid rate of change in technology, my friends and I tend to take for granted that we'll live through at least one more singularity.
But, given the very nature of a singularity, it's difficult to write past one. So we write not singularities, but the kind of disaster that comes from attempts gone wrong; anarchy, oppressive regimes, and accidental genocides. It's half adventure and half thought experiment as to how radical a shift we the writers could survive.
And it's a lot of fun to write.
All of it, if we ever finish, will be available free online under Creative Commons, because a communist approach to intellectual property is something else we four share. As one succinctly put it, it's better to have it out there for free and have people read it than to charge and sell one copy. In an ideal world, of course, we'd be able to exist on our writing and other people would be able to read it whenever they liked, but we don't have one yet.