Scirus is a search engine which currently indexes 545 million science-related web pages including .edu, .org and .gov sites.  A useful option offered by Scirus is that search results can be exported to a citation management program.  Results can be filtered by source such as PubMed or ScienceDirect.  They also can be limited to patent results via Lexis Nexis or to theses and dissertations from various university repositories.  File types is another filter available so you can limit to PowerPoint documents, PDF or HTML.

One of the strengths of Scirus is that it is able to filter out non-scientific sites.  Scirus is considered a very rich search engine because it draws its data from a huge repository of scientific sources.  The video describing Scirus is less than 6 minutes long.