Backuppc is not quite a "client server" application as you initially wanted, as it does not have a requirement to install anything on the client, in most cases. However it will backup both Unix and Windows clients.
Amanda might fit your bill for client-server type application, but depending on your specific needs, backuppc might just be what you are looking for.
You asked how to download?
Here -
http://sourceforge.net/projects/backuppc/files/
You need the
BackupPC-3.2.0beta0.tar.gz and you need a Unix box with perl installed. Most Unix boxes have by default. It will require samba libraries to backup Win32 clients.