Dropbox-like client for Macs to access existing FTP/WebDAV/CMIS storage
You may want to look into the following open source applications:


FTPSync

a Perl script to synchronize a local directory tree and a remote FTP directory tree.
ownCloud

Web services under your control. 

Current features: file management, WebDAV access, share via public link, music streaming, users &amp; groups, OpenID, LDAP
In development: sharing, encryption, calendar, contacts, etc., desktop sync client, Android &amp; webOS apps, server-server sync

NOTE: You do not have to utilize their storage.