What tools are useful to clone a linux box into another machine ?

If you mean clone system for your backup in case something happened during upgrade, I recommended to take backup from /etc which contain files system, if something happened for the file system you can copy back or you can use Image application like Acronis and make your full image if something happened just restore it There UNIX /Solaris command called dd it can do image for you too

fresh install is always better, but as Abdulqader said, you can create an image of the whole disk or partition with a tool like dd.
here's an example on how to use dd for cloning a partition: dd if=/dev/sda1 of=/mnt/sda2/sda1.img conv=sync,noerror bs=64K Note that the img should be placed on a different partition which is already mounted on your system, otherwise you will stuck in a loop!

you can use also clone zilla for more info http://clonezilla.org/

Any one you can choose accord your need and facility :

A.) Third Part cloning tools/ utility :

1.) Linuxghost - Mostly recommended and good for recovery.

2.) Clonezilla - Similar functionality to Symantec Ghost.

3.) PartImage - Compressed image file backup for Partition.

4.) PING (Part image Is Not Ghost) - similar functionality to Symantec Ghost.

5.) Mondo Resuce - Also good tools provides backup solution.

 

Traditional / Default & other available tools at OS level :

# dd - Disk compressed Or image backup.

# tar - Compressed backup for selected binaries (file/s & folder/s).

# Dump / restore - Provides the incremental backup solution.

# rsnapshot - Local & remote filesystem snapshot.

#rsync - Can be used to copy remote file/s.

 

Other Client / Server Solutions are :

1.) Amanda

2.) Bacula

3.) Symantec Backup Exec

Mondo Rescue is an open source, free disaster recovery and backup utility that allows you to easily create complete system (Linux or Windows) Clone/Backup ISO Images to CD, DVD,Tape, USB devices, Hard Disk, and NFS. And can be used to quickly restore or redeploy working image into other systems, in the event of data loss, you will be able to restore as much as entire system data from backup media.

Mondo program is available freely for download and released under GPL (GNU Public License) and has been tested on a large number of Linux distributions.

Google.
:P Seriously, I don't recommend cloning.
Just a fresh install if you have a new box.

Although  I don't recommend cloning. 

but you can use 

Acronis True Image

Always a fresh install is better. Anyhow we Have dd ,clonezilla a third party tool.We have to save the .img file into another partition safely.

CloneZilla is good to clone a Linux system. Or use rSync to backup your data and then install afresh.

dd linux tool

linuxghost is best for clone the linux machine