AppleJack gets you out of trouble!
As reliable as OS X can be, some basic maintenance can still be required. And not always the fault of OS X, an application can have a glitch too. In situations where OS X won't even properly startup - AppleJack can come to the rescue.
Before I started using AppleJack, when my OS messed up (usually after an update or my own stupid action) - my only option was a reinstall of Tiger. For whatever odd reason, this seemed to be the case on regular basis from about 10.4.3 to 10.4.5...and stopped with 10.4.6. Regardless of that - that which was at fault I found AppleJack was able to fix and save me that OS reinstall hassle!
In addition to getting you out of corner, AppleJack is also a great way to perform standard maintenance as well. Once installed, you can run AppleJack by first booting into single user mode (hold the 'command' and 's' key when you boot). At the command prompt, run can then launch AppleJack for options to repair disks, permissions, cleanup cache, validate preferences (removes bad), and remove swap files(can improve performance). If your system is acting funky or not even letting you fully boot up, it's very likely that using AppleJack will bring things back to norm.
Do note - you must install AppleJack _before_ you have a problem. Once installed, it will be waiting there at your beckoning should you choose to use it for preventative maintenance or for a real 'crises'.
(note: the following is outdated - AppleJack is now supported for Intel systems) At this time, AppleJack is available for PowerPC based Macs only. But, Intel Mac owners despair not - a version for those too should soon be in the works. BTW, AppleJack is FreeWare with no cost to you at all.