Last semester in one my MBA courses a couple of us students had to work on a couple group projects. Normally the group projects require us to create a collaborated document. One student knew of a program called Dropbox that allowed everyone to work on their part of the written document at their own time, yet have it readily available for everyone else in the group.
Dropbox is specifically designed to sync your selected files on all your computers (Mac, Windows, and/or Linux) and even your smart phones. Dropbox also allows the creator of the shared folder to designate who is able to view the newly created folder. This means you can share multiple folders to numerous different people and the users only see their allowed folders.