refseek.com – Academic Resource Search. More than a billion sources: encyclopedia, monographies, magazines.
worldcat.org – a search for the contents of 20 thousand worldwide libraries. Find out where lies the nearest rare book you need.
link.springer.com – access to more than 10 million scientific documents: books, articles, research protocols.
bioline.org.br is a library of scientific bioscience journals published in developing countries.
repec.org – volunteers from 102 countries have collected almost 4 million publications on economics and related science.
science.gov is an American state search engine on 2200+ scientific sites. More than 200 million articles are indexed.
base-search.net is one of the most powerful researches on academic studies texts. More than 100 million scientific documents, 70% of them are free