From Crossroad Solutions:
LooseStitch lets you create and share outliner documents. Brainstorm ideas, jot them down, call friends over, get feedback, make changes, and more. Create outlines for whatever you want, may it be cooking recipes, essay writing, presentation notes, sitemaps for Web designers, functional requirement documents, organising your ideas, managing your todos and tasks, and anything else you wish to keep track of.
LooseStitch uses the industry standard OPML file format to import and export outlines that can be used by programs like OmniOutliner. So if you have created outlines outside of LooseStitch, you can very easily set one up online by importing your existing file, with the assurance that you can always carry your work back to your outliner of choice.