Developer's Note: AppleJack has stopped being supported and has not been supported in years. It should not be used on any system after Mountain Lion.
AppleJack provides a user-friendly troubleshooting tool for Mac OS X. AppleJack allows you to troubleshoot your computer even if it isn't loading the GUI or you don't have a CD. AppleJack is menu-based and runs in Single User Mode.
AppleJack allows you to repair your disk, fix permissions, validate your system's preference file and remove corrupted cache files. These operations can most often help your machine get back on track. You don't need to bring another startup disk. You can simply restart in Single User Mode, (SUM), by holding down command and s keys at start, and then typing applejack or applejackauto (which will run through all tasks automatically) or applejackauto restart (which will also restart your computer automatically at the end).