frankschmitt.org:

Safari has this handy feature where you can select a word and right click to bring up a contextual menu, one of whose options is “Search in Google.” Trouble is, you lose whatever page you were browsing to the new Google search.

Well, the engineers at Apple thought of that, and if you have tabbed browsing enabled, just hold down the Command key when selecting this item, and your Google search will happen in a new tab behind the current tab.