AppleJack is a really handy troubleshooting utility for Mac that works even if you can’t load the Mac OS X GUI. Running in Single User Mode, it’s a simple menu-based tool to run a series of tests and repairs on the Mac OS. With AppleJack you can easily:

* Repair Disk
* Repair Disk Permissions
* Validate System Preference files
* Clear Caches
* Clear Swap Files

These are common Mac troubleshooting techniques, and really make AppleJack a handy utility in any Mac users toolkit. To use AppleJack, all you need to do is restart your Mac in Single User Mode by holding down Command+S during system start, then type applejack at the prompt. You’ll then be brought to the same menu you see in the screenshot above, where you can perform the necessary actions to get your Mac back in good health.

AppleJack is open source and free to download, highly recommended.