Meta-search engines compile and sort results from multiple search engines.
Dogpile Meta-search engine which brings together top results from Google, Yahoo!, and Bing
AllPlus Meta-search engine which compiles top results from search engines like Google, Yahoo!, MSN Live, and Ask.com
Content-specific Search Engines
Google Scholar Google's search engine for scholarly sources
Subject Directories
Subject directories are resources for specific topics.
Academic Info Online Subject Guides Large collection of subject guides for academic disciplines and specific research topics - list both online resources and printed reference works.
Internet Public Library (IPL2) Collections of resources from the Internet Public Library (IPL) and the Librarians' Internet Index (LII).