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 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.