Make Hidden Application Icons Translucent in the Dock
Notice anything different about that Dock? Some of the icons are translucent, and that is because they are hidden apps. You can enable this great feature yourself by entering the following command into the Terminal:
defaults write com.apple.Dock showhidden -bool YES
Then you have to kill the Dock for the changes to take effect
Now anytime you hide an application (Option+Click on the Desktop to hide the current application) the icon will turn transparent in the Mac OS X Dock. You can reverse this setting and have the icons visible as usual hidden or not by typing:
defaults write com.apple.Dock showhidden -bool NO
Then you must kill the Dock again for the changes to take effect and hidden app icons will no longer be transparent.
Thank you to Tyler Harden for sending in this tip!