Scholar, Google
Google Scholar provides a simple way to broadly search for scholarly literature. From one place, you can search across many disciplines and sources: articles, theses, books, abstracts and court opinions, from academic publishers, professional societies, online repositories, universities and other web sites. Google Scholar helps you find relevant work across the world of scholarly research. (Source: Google Scholar)
Link: http://scholar.google.com/
Tutorial: http://scholar.google.com/intl/en/scholar/refinesearch.html