In my first critique group, we'd write collaborative stories. I'd get something fairly vanilla and add a dead body over the side of the boat or a guy who stalks his house ninja-style to avoid looking at the crucifixes his dead mother hung in every room or two people who would instantly feel that bow-chicka-wow-wow. Inevitable, really, considering the direction my writing took.
I still love the freedom of a collaborative writing exercise. Apparently, I'm not the only one. The web has added infinite dimensions to those pass-the-spiral notebook days. Here's a list of some collaborative sites if you need that break from your work-in-progress or just want to feel inspired in the company of other writers:
Storybird
LOVE this one. Storybird focuses on picture book collaboration which makes it a perfect way to connect with the little ones in a writer's life. Storytellers browse collections of artwork, select the art they wish to use and craft text for each page. Stories can then be shared publicly on-site or privately with friends and family online.
Protagonize
Protagonize is a collaborative writing community where you can fly solo and post installments of your story or invite others to branch characters and plot lines in new directions. Stories are grouped by genre, and if you create a log-in you can be sure to organize the site to follow your favorites.
Ficly
Dedicated to the art of microfiction, writers can jump into anyone's story or begin one of their own in 1,024 words or less.
Bibliofaction
Bibliofaction champions the short story format and offers some great prizes for challenge winners. Find a short story by genre and "back cover blurb" or post your own and welcome feedback.
I really have tried that passing the notebook collaboration and to be honest I suck at it. I have friends who can make the pages fly with absolute clarity and come up with a great piece of work...but I really can't get far enough outside my own head and far enough into someone else's head to write with them.
I've done several of these kinds of things but honestly I generally don't enjoy it. I'd much rather be responsible for all or none of my own writing then mix my work with someone else. It's also, possibly, because such collaborative efforts aren't usually publishable.