Legendary Author Battles is a Google+ community started by Simon Cantan. It is a community for authors, who may or may not themselves be legendary (I consider myself more apocryphal, actually) but who want to sharpen their skills by creating short fiction with other authors.
There is a sheet of guidelines for creating projects, but the quick and dirty breakdown is that two authors get together to create characters and a setting and take turns adding to the story. These “battles” are intended to give authors a chance to work with other authors and to explore styles and themes that they might not ordinarily consider.
I am currently involved in two projects, very different from each other in mood and tone, and am really enjoying myself. I’d like to extend an invitation to other authors to check out the group. There are a number of open challenges posted (including mine) feel free to join in the fun.
mishaburnett 
Signed up. Not sure how much I’ll be able to participate though.
I should have added more to that. I’ve always been intrigued by a co-writing exercise, but have never found anybody to follow-through on one with me.