As you know, under the hood of that friendly Mac OS X system you're using lies the heart of a Unix machine, and the ultra powerful command-line that goes with it. For first time command line users it's a strange and mysterious place where dragons be, but long time users know the command line can help you get things done in ways you just can't work within the GUI. It's also a place to tweak settings not accessible from the GUI layers above.
Mac OS X Tips put together a really useful list of 15 hidden things you can change at the command line
. From un-hiding icons to changing annoying defaults in iTunes, this list
surely has some scratch for a nagging itch you have.