Linux only: Back In Time, a Linux backup app inspired by Macs' Time Machine and offering the same kind of no-worry, space-saving snapshot protection, is worth adding to your must-install list.
Why? First off, the user interface and documentation at its home page beat out similarly Time-Machine-inspired backup apps like previously mentioned Flyback, at least for my money. Second, it's smart about being friendly with non-developer Linux users—it rewrites all the rules of the underlying rsync command in plain English in its settings, installs a "root" launching shortcut for when you want to back up your core system files, and adds itself to your cron automation engine automatically at the intervals you set it to, no crontab -e or other terminal jobs required.
More than anything, though, the settings just make sense. You set a custom automatic backup wait in hours, days, months, or even years, and set Back In Time to forget about backups that are however old with the same kind of dialog. Because it's creating multiple snapshots with only the newest stuff in each, it saves you a good deal of space, but if you're running out—and you can set a limit on how much should be left—the Smart Remove function starts killing out old stuff, but keeps a good roster of backup points, as pictured at right. Just as important as the backup, Back In Time's restore functions are pretty easy to activate and understand—select a snapshot from the left-hand menu, choose folders from it, and hit "Restore."
Back In Time is a free download for GNOME or KDE-based Linux systems. Auto-updating repositories for Ubuntu and Fedora systems are posted at the Downloads page.