Its not commonly used, but I find WebDAV to be one of the more useful file transfer methods on the internet. It is secure (as secure as SSL can be), it can be accessed via web browsers, and drives can be mapped to it.
The only problem is that (under Windows) it is normally access by a user profile, and not the entire system. This can be a problem if you are trying to automate copy jobs or backups to the WebDAV share.
I found a great post at http://www.vnext.be/2010/05/20/powershell-upload-file-to-webdav-server/, that shows how to use PowerShell to copy to WebDAV. Its not an entirely elegant solution, but it can be scripted and user/password credentials can be stored to allow automated tasks to access it.