I've used ExpanDrive for a while- it lets you add alternate sources as drives in your save and open dialogs (ftp/sftp/s3/webdav), and I've been very satisfied.
Now they've introduced
StrongSync, which is a sync program using one of those same sources.
A couple of talking points from the site:
Control your data
Strongsync lets you connect to your own SFTP server or Amazon S3 account so you stay in complete control of your data.
Built on open standards
- SSH (SFTP), Amazon S3 and Dreamhost DreamObjects support.
- Rackspace Cloud Files, Openstack Swift and other backends coming soon
- SSH keys support
- Encrypted connections via SSH and SSL
- Deduplicated content-addressble data storage
It's available for Mac and Windows, with Linux support coming soon.
I'm downloading it to give it a try... it's $29.95 until years' end, and they have a fully functional trial version that is limited to repositories less than 1GB or 10,000 files.
