[Carbon Copy Cloner](http://www.bombich.com/) is one of the best applications for this. It is free and makes a complete backup of your hard drive to an external.

[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.