Address bar in Safari gone!
Right click in the empty area > "Customize Toolbar" > drag the address bar back up.
You probably accidentally command-dragged it out.
With the introduction of Safari 6.0, the command key sequence changed for this from command + shift + | to command + option + T.
It's also the first item under the view menu, 'Show Toolbar' or 'Hide Toolbar.'
edit: in Safari 7.0 and OS X 10.9, this feature is still available through View > Hide Toolbar or View > Show Toolbar, however there is no command key sequence.