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.
M. Vidal, A. da Silva, E. de Moura, and J. Cavalcanti. Proceedings of the 29th annual international ACM SIGIR conference on Research and development in information retrieval, page 292--299. New York, NY, USA, ACM, (2006)