Closed down a tab you needed in Safari? In the latest version, you use the same keyboard shortcut you'd use to undo an oopsie in any other program to bring it back. That's right — Command+Z (Control+Z on Windows) to the rescue.
Photo by scanlime.
It makes sense, of course, but it's also different from the Command+Shift+T (Control+Shift+T) standard that's emerged on Firefox and Chrome browsers on Mac and Windows. Still, knowing this shortcut should save you some time from looking for an extension to get the same job done.
Undo a closed tab in Safari 5 [Finer Things in Mac]