Two years ago, Google had launched a search feature for filtering search results by sites you have not visited yet. The option showed up on the side bar, most recently, under "more search options." But the option seems to be gone now.
Here is a picture from Search Engine Land of what it looked like when it launched - do note, it has changed over the years.
But now, on some browsers, for some users, and I can replicate it, it does not show up at all.
The first report I've seen of it going missing was two days ago at the Google Web Search Help forums. Sarah from Google confirmed the bug, saying "I could reproduce on Firefox 7 and will look into it."
There is no ETA for a fix or if this feature has been removed forever.
Forum discussion at Google Web Search Help.