synchronizing program? what to use and where to get it

im looking for a good synchronizing program that allows me to back up a bootable version of my mac on an external hard drive. any ideas what software to use and where to get it?

thanks

Hi, cardboard.

IMO SuperDuper! is the best backup software there is.

thanks looks sweet so if i back up my HD then shud anything go wrong that cant be fixed i will able to copy my HD from my external to my next mac and hey presto be back to normal?

That's what I use right before making an update. All updates are good at this point but I am ready if anything should go wrong with the installation or if the update is wonky.

But if you plan on using the backed up version on the external hard drive to install a newer version of the OS to another Mac (I am understanding this right?) it might not work because there could be bits in the OS that are model specific, depending on how the OS was installed on the first machine.

If it's only to synch files, you can always use target disk mode.

thanks i think a combination of these is just what i need