AppleJack Mac Troubleshooter

I haven't found myself in the position to need this yet. But, the Open Source AppleJack...

AppleJack Project Page

...sounds like something I should learn more about... just in case. It drops you into Mac OS X's Single User Mode text interface (very familiar to UNIX related OS users) where you can access critical parts of the system to fix hard drive , permissions, caches, and swap file problems.