dropbox
Dropbox was a service to upload a directory into the cloud and share files and directories with whoever you want. Now they also have a real time editing tool that includes a lot of features to add other type of media like videos, sounds, etc. including the basic commenting system. This probably built on top of the hackpad, service that they acquired 3 years ago and now you can import to DropboxPaper.
Pros
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Easy to use
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Web and local interface
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Powerful real-time editing with their DropboxPaper
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Convert DropboxPaper document to presentation
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Code and latex formatting.
Cons
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Conflicts difficult to understand (mostly with files that cannot be
diff-ed) -
Need account to comment or edit but not to view or download
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Dropbox Paper needs account to view and edit
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In Dropbox Paper a list of unknown contacts appears when you use
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