In Safari, we all know if you highlight some text and then Control+click (or right mouse click) you get a pop up contextual menu. I use this to search text in Google all the time, but didn't like that it would open in front while I was still reading the web page. So with a little experimenting, if you hold down the Command key before you click on 'Search with Google' in the pop up menu, the page will open in a tab behind your current one.
[crarko adds: I tested this, and it works as described.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20110628092358710