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.
Pros: AppleJack is the Swiss-Army Knife of emergency Mac repairs. With AppleJack, you will be able to repair your disks, repair permissions, cleanup cache files, validate preference files, and remove swap files, technically without having to boot your computer.
Cons: Buggy releases could cause some damage to your computer, but the developer is very quick to respond to serious bugs.