Top 15 Terminal Commands for Hidden Mac OS X Settings
There is a huge amount of hidden settings for Mac OS X and its applications that aren't accessible from preferences dialog boxes or the System Preferences. Applications such as Tinkertool and Mac Pilot allow you to access some of these, but the real flexibility is from the Terminal. From here it is possible to edit any preferences file for any application on your Mac.

You'll find the Terminal in the Utilities folder within the Applications folder. To carry out any of the following commands you will need to copy/paste or type in the line of text then hit enter. For the most part, applications will need restarting before changes take place. For most applications you can just quit and open them again, and for the Finder you can use the Force Quit dialog, just log out and log in again or type "killall Finder" into the terminal after the command.

Feel free to add any of your favourites in the comments.

1. defaults write com.apple.Dock showhidden -bool YES
Makes hidden applications' dock icons translucent. NO to reverse.