Automatically backing up

[Yeahman]

I no longer have my desktop so I'm using my laptop exclusively. I want my files to be automatically backed up. I figure an external hard drive is the best option. I can also use it for extra storage capacity. But how can it backup my files without having to manually transfer them?
When I had my desktop, I kept everything on the desktop and made them available offline on my laptop. It would sync everything I started up or shutdown. Something like that would be nice.

[hooligan]

If you're running XP, there's a powertoy that'll autosync.

http://www.microsoft.com/windowsxp/using/digitalphotography/prophoto/synctoy.mspx

[didu]

rsync http://samba.anu.edu.au/rsync/

[bluemonq]

I no longer have my desktop so I'm using my laptop exclusively. I want my files to be automatically backed up. I figure an external hard drive is the best option. I can also use it for extra storage capacity. But how can it backup my files without having to manually transfer them?
Most external drives come with automated backup software.