ownCloud is a very well supported open source file sharing application: a private alternative to iCloud, various Google services and the like. There are native synchronization apps for all of the usual phone and desktop operating systems; as well as all sorts of third party stuff that works using the webdav, caldav etc standards.
I have it set up here for file sharing and synchronization, calendar, and contacts. If I know you and you would like an account, you know how to reach me.
My private server is here.