Scirus, the Elsevier-produced web index targeting the retrieval of over 380 million science-related Web pages, has always been a credible free alternative to the imprecision and paid-advertising that comes with attempts at scientific retrieval with basic google. (of course, Google Scholar is where you could go for better peer-reviewed scientific access).
It is important to recognize that Scirus is not exclusively indexing open access resources. Scirus offers search of MEDLINE, Science Direct, the Nature Publishing Group, and a number of commercially published scientific society journals, such as the American Physical Society(APS), LEXIS/NEXIS patent materials. The open access coverage in Scirus stems from an international array of open access journal publishers, institutional repositories, and scientific societies:
• 600,000 e-prints from ArXiv.org
• 57,700 full-text articles from BioMed Central
• 23,300 documents from Caltech Coda
• 3,400 e-prints from Cogprints
• 87,100 full-text articles from Crystallography Journals Online
• 28,600 documents from CURATOR
• 2.5 million documents from Digital Archives
• 21,400 documents from DiVa
• 105,000 full-text articles from Project Euclid
• 6,000 documents from HKUST Institutional Repository
• 59,700 documents from The University of Hong Kong
• 20,500 full-text documents available from IISc
• 13,400 full-text documents available from Humboldt Universität
• 314,000 full-text articles from Institute of Physics Publishing
• 19,400 full-text articles from Maney Publishing
• 75,900 documents from MIT OpenCourseWare
• 25,200 technical reports from NASA
• 876,000 full-text theses and dissertations via NDLTD
• 10,400 documents from Organic Eprints
• 1,890 documents from PsyDok
• 1.7 million articles from PubMed Central
• 884,000 documents from RePEc
• 18,100 documents from University of Toronto T-Space
• 151,000 full-text documents from WaY
The simple Scirus search interface performs an initial retrieval and will offer a result filter where a searcher can select the open access resources. Take a look at this search for West Nile Virus.
So the search filter returns a mixed result, along with this filter column to the left. Seems like the open access material is under the Preferred Web category, along with the journal sources PubMed Central, Hindawi and BioMed Central.
I want to root for Scirus because it does benefit open access by exposing researchers to that no-cost preferred web content, at least until someone has to whip out their credit card and pay for an article because they or their institution does not subscribe.