What level of backup are you looking for? rsync ( http://www.samba.org/rsync
) is a great free tool to back up just about anything but it takes a little finess to get it setup just right. Rsync is one of my favorite solutions for things like web servers where you just wanna back up html, conf, txt, etc. and then only back the up again when the files have and you're not really worried about the binaries.
There are also a lot of great open source solutions that resemble of an enterprise backup solution. For example, I like Amanda its easy to use and has great documentation. These are good for full os level backups or database exports. Check it out at http://amanda.zmanda.com/
Alternatively you can do full image backups with something like Clonezilla ( http://clonezilla.org
)or Acronis (a paid solution). Clonezilla is great but its cold cloning (meaning there will be downtime just like when you use your live cd).
Depending on what the system is most often I use just an rsync back up and move it to a redundant system so in the event my main server goes offline the back up server can be online in a matter of minutes. I do however take OS level backups (usually images with Clonezilla) before and after patching systems.
Hope that helps! There's also a ton of information on LQ and google that address back ups and best practices depending on your specific environment and system purpose.