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Google Co-op is a platform provided by Google that allows web developers to feature specialized information in web searches, refine and categorise queries and create customized search engines, based on Google Web Search. Google launched the service on May 10, 2006.
Google Co-op platform consists of three services.
Custom Search Engine
Released on October 24, 2006, Google Custom Search allows anyone to create their own search engine by themselves. Google adSense results can also be triggered from certain search queries, which would generate revenue for the site owner. This service is currently being beta tested, with bugs, enhancements and feedback from developers being recorded at the "Google Custom Search Engine Enhancement Page".
Subscribed Links are web results that users can manually subscribe to. Anyone is allowed to make a new Subscribed Link, and do not necessarily need knowledge on how to create a feed, as a basic link can be created. Subscriptions are then available in a special directory. 
Topics are specific areas of search, which can be developed by people with knowledge of a certain subject. These topics are then displayed at the top of relevant Google web searches, so the user can refine the searches to what they want. Currently, there is a limited number of topics that Google is wanting to develop, namely Health, Destination Guides, Autos, Computer games, Photography and Home Theater.
One of the topics with many contributions is Health. They include the National Library of Medicine, Centers for Disease Control and Prevention, Health On the Net Foundation, Harvard Medical School, Mayo Clinic and others.
A typical example of custom search engine is AllGameSearch . This is the google powered custom game search engine exclusively for game industry.