You may sincerely wish never to need it, but if you run into a situation where your Mac refuses to boot fully to the Finder and you don't have any boot-capable utilities to tackle the problem, Kristofer Widholm's AppleJack is probably your best friend in the trenches. The single-user-mode tool is only available if you start your machine while holding down Cmd-S to boot to the command line (as per Dave's tip from yesterday), but once there it streamlines the process of checking your disk for errors, repairing permissions, cleaning caches, zapping corrupt preference files, and deleting leftover virtual memory swap data. You could do most of this yourself via a series of commands, but AppleJack is a) much, much simpler and b) completely free and open source.
For a long while it didn't look as though AppleJack would make the leap from Tiger to Leopard, as Mac OS X 10.5 made some key changes that broke AppleJack's functionality (specifically, the 'id' command didn't work, and the /etc/rc file was removed). Developer Steve Anthony jumped into the project this summer and was able to code around the Leopard-specific problems, finally giving us back our beloved AppleJack in version 1.5.
Let's all raise a glass of the original Jersey Lightning to Kristofer and Steve for their work on behalf of Mac troubleshooters everywhere.