services providing Internet-accessible space for your files (PayLoad), either for your own use/Back Up/Data Synch, or for File Sharing - your online Hard Drive
WebDAV is a good protocol to support
comparison link - 2006-02-01-OnlineStorage http://www.flickr.com/photo_zoom.gne?id=93730415&size=o
Sept'2013: Ben Brooks is minimizing his Drop Box usage (because of the NSA) and playing with BitTorrent Sync, Own Cloud, and File Transporter)
can't get to it if you're Off-Line? Some of them do Data Synch to your local drive - downside (if using for File Sharing) is that everyone ends up with a huge copy of stuff they rarely need locally.