Have you ever had the experience, when searching for an item on Google, that a friend receives a different result list than you - or maybe different results altogether?
This kind of search difference got Eli Pariser, director of the progressive US political website MoveOn, thinking about how search personalisation can limit our world view. He wrote a book and gave a TED talk recently, on just this issue.
The example he gives which best illustrates his concern, is the differing search results Google returned for the exact same query to two different people. When Egypt was entered, right around the time of the Egyptian protests, one friend received results centred around the political upheaval, while the other received travel information.
The TED talk is only 10 minutes, so if you can spare the time it's well worth a watch.