Time Machine does it. Either hook up your external drive with a USB to your Mac or to an airport (if you have one) to back up wirelessly.
It will back up your computer every hour, every day, until the backup drive is full. Once it gets to this point, it will notify you that it is full, and that it will start deleting the hourly backups starting from the oldest (which you usually don't need by the time you've filled the drive) to make room for new backups.
Highly recommend it, I've accidentally deleted things between hours and it helped me recover things.