Keyboard Kung Fu: Command-Q = Quit application
You’ve probably noticed when you close an application by clicking on the little ‘x’ at the top left of the window that this doesn’t completely close the app. You can tell that the app is still running by the little arrow beneath the app’s icon in the Dock. Completely closing the app is known as ‘quitting’ the app. To quit the current application, simply hold down the Command key and tap on the ‘Q’ key: ⌘ Q
When Lion (10.7) was released it introduced a new ‘resume’ feature that adds the ability to restore the state of files and windows of an application as they were when the program quit with ⌘ Q. If, however, you don’t want the program to save and restore its last state, you can now include the Option key while quitting, eg: ⌥ ⌘ Q and the previous state will not be saved.
Combine Command-Q with Command-Tab
I often ⌘ tab to see what apps are still running that I am no longer using and then ⌘ Q to stop them running, freeing up system resources.