I saw this list of search engines and knowledge base websites. Add any that you've come across to this thread.

www.refseek.com - Academic Resource Search. More than a billion sources: encyclopedia, monographies, magazines.

www.worldcat.org - a search for the contents of 20 thousand worldwide libraries. Find out where lies the nearest rare book you need.

https://link.springer.com - access to more than 10 million scientific documents: books, articles, research protocols.

www.bioline.org.br is a library of scientific bioscience journals published in developing countries.

http://repec.org - volunteers from 102 countries have collected almost 4 million publications on economics and related science.

www.science.gov is an American state search engine on 2200+ scientific sites. More than 200 million articles are indexed.

www.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.

imslp.org/wiki/ is a database of sheet music and scores.