hackpadhttps://hackpad.com/

It’s kind of like GoogleDocs, but better for collaboration. You can invite tons of friends to brainstorm with you… or set your docs as private. It’s your choice. I’m stuck on WorkFlowy, so I might not use this as my primary tool, but it still looks interesting. If you try it, please let us know your thoughts here. (And thanks for the tip, Caleb!)