AppleJack

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).

• Snow Leopard compatibility [feature 2845796] (Thanks again to Steve Anthony)
• Improved limits on output from syslog to STDOUT
• Simplified startup of services on Leopard and Snow Leopard
• Fixed bug in creation of user account lists in Snow Leopard where system accounts would show up
• S.M.A.R.T. status verification is now being done in the expert mode. I still want to implement this using smartmontools, but for now diskutil will do.
• Blessing of Mac OS X System folders on attached volumes is now possible. This is a primitive bless, ie, it does not create boot files, but simply blesses the chosen System folder and (optionally) sets it to be used for startup on next launch