Google's just launched "Google Scholar", a new search engine looking up within scholarly literature, including peer-reviewed papers, theses, books, preprints, abstracts and technical reports from all broad areas of research.
This new service is the result of the Google's collaboration with a wide variety of academic publishers, professional societies, preprint repositories and universities. In addition, it returns as well a lot of scholarly articles available across the web.
http://scholar.google.com/