There are many applications for helping you back up data, including Carbon Copy Cloner, Leopard's Time Machine, and MobileMe's Backup. Yet most of these aren't full featured, easy to use and come as part of other software and services.
For those who want more power and ease of use, have Tiger and don't want to pay a MobileMe subscription each year, Get Backup is ideal. With version 2.0, it includes file synchronisation functions for making sure two different Macs are up to date.
Get Backup is simple to use, with buttons for backing up common files, such as iTunes and iPhoto libraries, Mailboxes and Address Book. There are options to compress data into standard .tar archives, to back up in increments and to maintain old versions of files. You can also schedule backups to run automatically and filter out files by extension, file name and other patterns.
New to version 2.0 is file synchronisation. If you work on two Macs, making sure they have the same, most up-to-date versions of the files you're working on is important. Get Backup makes a good attempt at file synchronisation, offering one-way and two-way synching, and it tracks files properly: delete a file from one Mac and it'll delete it from the other, not create a new version the next time you sync.
It's very fast, although it crawls when dealing with many files or a 802.11g network. However, the synchronisation interface is confusing, and greying-out is the only sign that something might be deleted – the process can be scary if you don't have a backup: ironically, it doesn't make backups of files it has deleted. There is a don't delete files mode, but it will drive you crazy with 'immortal' files.
Although it's not absolutely best in class, for backup and synchronisation features at this price, it can't be beaten.