OS X Tip – Highlight Stack Items
Leopard has a little hidden feature in it where you can highlight stack items when you hover over it. By default, this feature is disabled. To enable it, simply open a Terminal window (Applications –> Utilities –> Terminal)
In the Terminal window, type:
defaults write com.apple.dock mouse-over-hilte-stack -boolean yes
killall Dock
To disable this feature, type:
defaults write com.apple.dock mouse-over-hilte-stack -boolean no