AppleJack is a great troubleshooting assistant for Mac OS X. With AppleJack installed, you can troubleshoot a computer even if Mac OS X won’t fully start, or don’t have a startup CD handy.
We recomend AppleJack in our tiny-but-popular My Mac Won’t Start! guide. We’ve been holding back this tip for a new version that is compatible with Apple’s new Intel-based Macs. Happily, version 1.4.3 which adds Intel-compatibilities was released this month.