www.crashplan.com
I've been using for months now to backup family pictures and stuff like that. I have it setup on a PC at home as well as my Macbook and even a buddy and I have played around with backing up to each other's computers...pretty slick if you ask me.
The initial backup takes a little while, especially if you are doing it over a WAN but if it's on your internal LAN then it's no issue at all. Afterwards, it just does changes as often as you like.
Edit: Oh, and it's free!!