The bootable Mac OS X CD-ROM is soooo important — try to have more than one copy of it around. That way, if one gets misplaced, damaged, eaten by the dog, scuffed, scratched, or otherwise rendered useless, you won't be out of luck. In addition to your Mac OS X CD, an older version of Mac OS and the CD that came with your computer are examples of extra bootable CDs you may have hanging around. All of the above will boot your Mac in an emergency, which is why they're so important to have handy.

If you have a removable media drive such as an Orb or Jaz or Zip drive, or a CD-R or DVD-RAM, create a bootable (removable) disk by installing Mac OS X or any of the others mentioned above your media and stash it someplace safe, just in case.