These days, my writing is mostly for fun, and I enjoy working on projects with others.
So, there are a lot of websites that do roleplay game type group writing and such, but sometimes I like writing with folks without using a system, just in round-robin type styles to see where a story will go. We do that kind of thing at http://www.lostpathway.com
I love it when a story becomes something beyond any one person's imagination through shared storytelling, and I also enjoy communal writing where one person doesn't have to "own" or "be" one of the characters, but the goal is to team up for an enjoyable, fun storytelling experience.
More system based stuff can be fun, too, and is welcome.