Duplicity performs incremental, encrypted backups onto a range of interfaces including scp, ftp, rsync, WebDAV and Amazon S3. It uses standard tar, rsync and GPG tools, so the data can be recovered manually.
JungleDisk is a WebDAV server which stores it's data on Amazon S3 (via a local cache). It's commercial, non-free software for Linux, Mac and Widows and costs $20, lifetime registration.
The equivalent of sshfs but for webdav. Mount webdav shares as local filesystems using filesystem in userspace (fuse).