Scirus is the most comprehensive science-specific search engine on the Internet. Driven by the latest search engine technology, Scirus searches over 480 million science-specific Web pages, enabling you to quickly:
• Pinpoint scientific, scholarly, technical and medical data on the Web.
• Find the latest reports, peer-reviewed articles, patents, pre prints and journals that other search engines miss.
• Offer unique functionalities designed for scientists and researchers.
http://www.scirus.com/
Google Scholar provides a simple way to broadly search for scholarly literature. From one place, you can search across many disciplines and sources: peer-reviewed papers, theses, books, abstracts and articles, from academic publishers, professional societies, preprint repositories, universities and other scholarly organizations. Google Scholar helps you identify the most relevant research across the world of scholarly research.
http://scholar.google.com/