A list of major metacrawlers. Unlike search engines, metacrawlers don't crawl the web themselves to build listings. Instead, they allow searches to be sent to several search engines all at once.
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X. Fu, J. Budzik, and K. Hammond. IUI '00: Proceedings of the 5th International Conference on Intelligent User Interfaces, page 106--112. New York, NY, USA, ACM Press, (2000)