Google has a cool feature called Google Search History. This is really cool where the user can sign up with their email address and log in using that email ID. Then all the searches that they do with that account will be recorded. This will help Google to enhance the search experience of the users by refining the seach results. But using this history, Google can maintain what every user is doing on the net. Every search that I do will be recorded and Google can use this database to create other services like Yahoo. Also if I search for any illegal things like cracks, warez, etc., I can face the wrath of the law.
See this link for an article on Google Search History http://www.oreillynet.com/pub/wlg/6914