If you like to keep your dock automatically hidden for more screen real estate on your Mac, it might bug you that it takes a good half second for it to show up when you mouse over it. This simple terminal command will eliminate that delay, so it shows up instantly.
This secret setting coincides with the "Automatically Hide and Show the Dock" preference in System Preferences > Dock, and it only works in OS X Lion. If you like that setting but don't want the delay, just open up a Terminal window and run the following command:
defaults write com.apple.Dock autohide-delay -float 0 && killall Dock
From now on, when you move your mouse to the Dock's screen edge, it should appear instantaneously. If you want to bring the delay back, just run:
defaults delete com.apple.Dock autohide-delay && killall Dock
Check out the above video for a quick demo of what this looks like. You can also change the delay length if you want, to make it longer. Hit the link to read more.
10.7: Remove the Dock's Display Delay | Mac OS X Hints