dropbox.com

According to wiki, Dropbox is a file hosting service operated by Dropbox, Inc. that offers cloud storage, file synchronization, and client software.

In brief, Dropbox allows users to create a special folder on each of their computers, which Dropbox then synchronises so that it appears to be the same folder (with the same contents) regardless of the computer it is viewed on. Files placed in this folder are also accessible through a web site and mobile phone applications.

www.dropbox.com

I installed dropbox in my computer, put sharing files into it, and sharing dirs/files with others in a synchronization way, very cool, and convenient!
To share files, the key is to install Dropbox software in each computer.