Bacula - The Network Backup Solution - From their website.

Bacula is a backup suite that has been designed to be modular and to scale from 1 machine to a whole network,

The following image shows how it all plugs together: bacula-applications.png

Bacula also has MicrosoftWindows client for the file server so can be used to backup a mixed network.

More information including great documentation can be found on the website and can be downloaded from Sourceforge.

Note this suite has a NewZealand connection, Nic Bellamy (from NZLUG) is one of the developers.