AppleJack is a user friendly troubleshooting assistant for Mac OS X. With AppleJack you can troubleshoot a computer even if you can't load the GUI, or don't have a startup CD handy. AppleJack runs in Single User Mode and is menu-based for ease of use.
Using AppleJack, you can repair your disk, repair permissions, validate the system's preference files, and get rid of possibly corrupted cache files. In most cases, these operations can help get your machine back on track. The important thing is that you don't need another startup disk with you. All you need to do is restart in Single User Mode (SUM), by holding down the command and s keys at startup, and then typing applejack, or applejack auto (which will run through all the tasks automatically), or applejack auto restart (which will also restart the computer automatically at the end of the process).... More Info »
xlr8yourmac.com has compiled a list of user experiences with AppleJack which might help you evaluate its usefulness to you. Thanks guys!
Dan Frakes has written a nice summary of the benefits and drawbacks to AppleJack for macfixit.com. It's thorough and much better written than anything I could have done.