Dan Frakes has a rather thorough article up over at MacFixIt, covering
the basics of OS X maintenance and recommending the command line tool, AppleJack,
as a viable troubleshooting tool. What makes it better than a lot of
other tools out there is that it can be used when you cannot even get
your machine to boot: "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."
Make sure you check out Frakes' full article for a more comprehensive discussion.