I especially appreciated Zamazar for its ability to translate file formats. This could be very useful for reducing file sizes such as turning a Powerpoint into a PDF. One use for document-sharing sites would be if I am working on a grant with other partners. This would enable us to have one official working copy, so as to avoid confusion.
I have used a productivity tool called "RefWorks" through the University of Minnesota Library system. This is actually a citation management system that can be accessed by numerous team members.