Like a fire extinguisher by your stove or a lead pipe by your front door, AppleJack can be extremely useful to have in the right situation. This free command-line utility can help you do a small suite of essential Mac maintenance tasks, without needing an OS X install disk or secondary startup disk.
Don't be intimidated by hearing the words "command-line": if you already have AppleJack installed on your machine when you start up in Single User Mode (the only time you'll ever want to use AppleJack, by holding down command-s when you restart), you'll see a prompt asking you if you want to use AppleJack "for troubleshooting assistance." Then you'll be given a user-friendly list of basic maintenance options, including repair disks, repair permissions, cleanup cache files, validate preferences files, and remove swap files. These are all things that you can already do with the command line, or that you can do with Disk Utility if you're starting up from a secondary disk--but if you're traveling or otherwise don't have your install disks with you, AppleJack can be a lifesaver for getting an ailing machine up and running again. AppleJack supports Snow Leopard, and recent updates have improved the start-up process.
Like that lead pipe or fire extinguisher, though, AppleJack is not necessarily recommended for recreational use. A small number of users on some systems have experienced issues running this app, so there's no need to run AppleJack unless you are already having problems--and as always, make sure you have a backup of your hard drive.