You don't really have any good reason not to back up your Mac with Time Machine built right in. Time Machine is Apple's integrated backup system for OS X, and all it requires is an external hard disk in order to work. Since OS X 10.5 Leopard was released, Time Machine has been available to back up Macs. The software works by incrementally backing up all the data on your Mac's hard drive, providing you with the ability to completely restore your Mac in the event of a catastrophic drive failure, or to selectively recover files you've deleted or removed in the past by entering the Time Machine interface and looking through snapshots of your Mac as it was in previous backups. Time Machine backs up hourly for the past 24 hours, daily for the past month, and weekly for everything older than one month until the target volume runs out of space. It's also designed to work with Apple's Time Capsule, a network router with integrated backup storage.