There are several commercial tools for cloning hard disks for OS X, including SuperDuper! and Carcon Copy Cloner. However, the same can be achieved by using rsync.

I wrote a script based on necolas' rsync_backup. It has several improvements over necolas' version, but the core is the same. I used Carbon Copy Cloner's list of files to exclude when cloning.

I tested the script on my machine using an external USB hard disk on OS X 10.7 and after cloning for a few hours (like the first Time Machine backup), I can simply boot off the USB disk by holding the 'Option' key. rsync can make incremental updates, such that next clones are much faster (~10 minutes in my case).

N.B. I do not guarantee anything for this script, if it breaks your computer: tough luck. There are some safe guards in the script, but I refuse to promise anything. I recommend to read through the script and verify that it works before use.