Google competitor DuckDuckGo reports that people searching for the same terms on Google can encounter very different results, because Google tracks details of your search history and which results you click on even if you're not logged in to Google. They've released a minute-and-a-half on-line commercial to illustrate the point. DuckDuckGo's big selling point is privacy, so this is a self-serving message, but that doesn't mean it's not a valid point.
Carl Franzen has a good article about this at Talking Points Memo that explains that it apparently is possible to get non-personalized results from Google, even if it's not easy. As Franzen notes, Google's documentation essentially backs up what DuckDuckGo says about Google's search personalization.
Search personalization is not necessarily a bad thing. For one thing, it's more apt to give you results you're interested in. But as DuckDuckGo points out, it also means that especially in the case of research related to political questions, you're apt to see results that support your preconceptions -- liberals tend to get links to liberal sites, conservatives to conservative ones. That's apt to reinforce our prejudices and make people even more politically polarized than they already are. It's worth thinking about.by