The use of the Altavista Yahoo!, AllTheWeb Sogou, Bing, and Google search engines should be acknowledged, too. Although none of the above are what one might wish, they are nevertheless instrumental in this mapping.
Updated in 2011 to account for Altavista's transition to Yahoo! servers, and in 2014 to reflect the demise of AllTheWeb. In 2019, the Sogou engine was dropped in favor of Bing due to difficulties collecting Sogou's results data.