ownCloud: The cloud sharing service you control

ownCloud is an open-source effort to put control back in our hands. It is still young and has rough edges, and has a way to go to achieve its ambitious goals, but it is already useful and exceptionally easy to use. You can easily set up and manage data storage and sharing, and shared calendars and contacts. The server runs on Linux, Mac OS X, and Windows, and it has clients for desktop and mobile platforms. There is an active and growing developer and user community, and improvements are coming thick and fast. This is the #1 cloud project to watch, in my occasionally humble opinion.