Mac OS X has a commandline tool called "defaults". It lets you set certain properties, for example:
enable the "Debug" menu in Safari:
defaults write com.apple.Safari IncludeDebugMenu 1
What is the most useful "hidden" "defaults write ..." command you have ever issued?
There is a database of these hidden settings at secrets.blacktree.com. They also offer a free PrefPane you can install to twiddle these (instead of cutting and pasting from the website into Terminal).
My favorites:
defaults write com.apple.Safari TargetedClicksCreateTabs -bool TRUE
The above forces all links to open in new tabs, instead of new windows.
defaults write com.apple.screencapture location "/path/to/save/screenshots"
This one saves your screenshots somewhere else, other than your desktop.
defaults write com.apple.screencapture type PDF
And this one changes the format of your screenshoots to PDF. I am sure some might find this one handy.
defaults write com.apple.dock showhidden -bool YES
ghosts out the icons of apps that are currently hidden.
Well, actually, the most useful I ever wrote was, IIRC:
defaults write NSGlobalDomain NSInterfaceStyle NextStep
but that doesn't do anything on versions of OS X with Aqua :-(.
defaults write com.apple.iTunes AppleDisplayScaleFactor .75
Shrink iTunes to 75% normal size. It's off-topic, and not really that useful (unless you're using a small display), but fun enough that I can't resist posting it. It works for other apps too, e.g. To shrink Calculator.app by 50%:
defaults write com.apple.Calculator AppleDisplayScaleFactor .5
defaults write com.apple.desktopservices DSDontWriteNetworkStores true
Disables the creation of .DS_Store files on Network Shares.
