Dropbox Paper is a nice free online service/tool to collaborate with others by saving documents, pictures, or other files in the cloud and share them with those working on the same project with you. Everyone in your team can modify the documents, or contribute to the project by adding ideas and files. Tasks can be scheduled and assigned as well so that this tool could also serve as a simple project management tool.
Some short feature description excerpts from the Dropbox Paper site:
Teams can work smoothly together in Paper and everyone can contribute to growing better ideas in a flexible workspace. Even when you are on the go you can use Dropbox Paper on iOS and Android mobile apps.
Track tasks with due dates and assign with @mentions. Keep everyone in the loop with @doc, without calling a meeting. Use Google calendar sync and meeting notes will be shared automatically with attendees, making it easier to run meetings.