You need hardware:
x a computer running a free unix clone.
x as much discspace as you can aford.
  
You need software on clients and server:
x ssh/openssh
x GNU bash
x GNU tar 
x GNU shellutils

How to prepare your computer:
x mount your huge partition (we use /backup).
x make it possible for ssh to log in from this computer
  to your other computers as root without a password. scary.
x put this software in a directory (we use /home/backup/)
x put the name of your computers in /home/backup/conf/computers.unix
x the fifth field (of the partitions you want to back up) in
  /etc/fstab on your clients should be >0 

Crontab entry:
We use cron to start the script every night. Here is the entry we use:
2 2 * * * /home/backup/backup.sh < /dev/null >> `/home/backup/logfilename.sh` 2>&1