<img src="https://i.sstatic.net/mudjv.png" width="40" />
[Carbon Copy Cloner](http://www.bombich.com/) is one of the best applications for this. It is free and makes a complete bootable backup of your hard drive to an external. I've used Carbon Copy Cloner many times and I highly recommend it.

<img src="https://i.sstatic.net/RYqxv.png" width="40" />[SuperDuper!](http://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html) is another option. It allows you to create a fully bootable backup of your hard drive. The backup part is free, so you don't need to pay the $27.95 for the scheduling part. I've used SuperDuper! as well and it is a great application.