Scirus
A search engine sponsored by the publisher Elsevier that focuses on science-specific web content.  Searches web information, preprint servers, digital archives, repositories, and patent and journal databases. 
http://www.scirus.com/srsapp/advanced/index.jsp

Google Scholar
A broad web search for scholarly literature in all disciplines. Sources include peer-reviewed papers, theses, books, abstracts and articles that come from academic publishers, professional societies, preprint repositories, universities and other scholarly organizations. Use the advanced search page to limit results to a specific discipline.
http://scholar.google.com/