Dropbox is a web-based file hosting service which uses cloud computing to enable users to store and share files and folders with others across the Internet using file synchronization. You can download it at www.dropbox.com
Dropbox can be installed on Windows, Mac OS X, Linux, iPhone, iPad, Android devices and more. After creating an account and installing, you will find a folder called "My Dropbox". All files in this folder will be synchronized seamlessly to every other device that has Dropbox installed. If you can't install the client, you can still use the web interface at www.dropbox.com - just log in with your username and password in the upper right corner.
Dropbox also offers a version history and sharing features. Using the version history you can easily revert back to previous versions. The sharing features are great for collaborating on files and folders.