Emergency Startup Mode, which allows you to reboot from your hard disk even if you throw away or damage the System Folder.
Installation is simple.
Rewind Disks lets you rewind your hard disk back to a time when it worked. Emergency Startup Mode is, to me, the best part. If your Mac won't boot, hold down the R key, press the Power button, and your system will start up in Rewind Emergency Startup Mode. Then you can simply Rewind your hard disk or System Folder to a time before you experienced problems. When you boot into the Rewind Emergency Startup Mode, you also can use any of your disk or anti-virus utilities to repair problems.