"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." Great for refreshing older Mac OSX systems.
"Apple has developed its own (better) implementation of cp. This command, called "ditto," not only copies the files from the source directory to the destination directory, creating the destination directory if it doesn't already exist; but will also merge the contents of the source directory with the destination directory if it does exist."