If you do a search some results will contain a ‘Search [website]‘ form right inline with the SERP result. A neat little feature that could making searching that much easier.

Give it a try by searching for Amazon:
http://www.google.com/…

Check out these posts too:

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  3. Easily add Custom Google search to your website
  4. Google Book Search searches magazines
  5. New Google flight search
  6. Google Begins Move to Universal Search
  7. Search Google like its 2001