Cooperatively?
Well, there's that Daigokuse (I spelled it wrong, didn't I...) no Ranma
series that has a team of writers on it.

And to some extent, the elseworlds series is a co-op effort overseen by
J.Biles, I think...

And then I think I'm gonna be killed by a tank in a fanfic written by
some guy named Ed.  No kidding.

anyways, those two series are some of the better stuff out there, so
co-op work I think is alright...

-rod