If you like working with whiteboards and/or index cards for your stories, you might want to check out Trello. According to their About page, Trello is "a collaboration tool that organizes your projects into boards. In one glance, Trello tells you what's being worked on, who's working on what, and where something is in a process."
To test drive the service I set up this example story board, which you can see in more detail here. Using Trello is pretty straightforward; you just have to fiddle with it a bit to get the hang of how to edit and arrange your cards. I'd be most likely to organize a story project outline as shown and keep a running list of what tasks I need to accomplish, but you can use your cards for whatever you like.
Two of the features I liked most are the simple color-coded tags you can apply to any card, and the search feature -- you can sort through everything by any keyword, which makes finding pertinent data and editing the cards a snap.
For those of you who collaborate with another writer Trello would probably be extremely useful. You could set up an account for the two of you, organize your schedule, plot out your next project, work out who needs to do what and update each other on your progress. The right activity sidebar shows all activity on the board in chronological order, so you can always look there to see what's been added, deleted or changed recently. You can also set the board to be private so that no one sees it but you and your writing partner.
Thanks to our blogpal author Charlene Teglia for originally blogging about Trello.