• BASE is one of the largest search engines especially for academic open access web resources. Operated by Bielefeld University Library. 
   

• CORE is an experimental service, smaller then BASE, but with more advanced discovery tools. Key feature: if you find an article you like, CORE will find similar ones by analysing the text of that article.
   

• Google Scholar is a is web-based scholarly search engine. To find freely available articles, enter filetype:pdf behind your search query. Also see Google Scholar Library Link

These free and simple to install browser extensions look automatically for open access copies of paywalled research articles. 

Unpaywall: an extension for Google Chrome and Firefox that searches several large open access databases.

Open Access Button: an extension for Google Chrome, Firefox, Internet Explorer and Safari. The Open Access Button searches several large open access databases. It also sends a request to the author to open up an article that can't be found for free.

• Scholarly Publications: provides access to scientific output of Leiden University.
   

• OpenAIRE: provides access to more than 8.000.000 of Open Access publications from 460+ data providers from 38 Eurean countires, with links to funding, datasets and more.
   

• Digital Commons Network: provides access to almost 800.000 scientific publications from over 300 (mainly US) academic institutions.
   

• Oaister: provides access to over 30 million records found by harvesting over 700 repositories and open access collections worldwide. It is integrated in WorldCat, but also available as separate catalogue.
   

• openDOAR: authoritative directory of academic open access repositories worldwide. OpenDOAR lets you search for repositories or search repository contents. It uses Google Custom Search Engine to offer full-text search of repositories listed in OpenDOAR.
   

• Registry of Open Access Repositories: similiar to openDOAR, but, in contrast to openDOAR, is filled automatically.

• Openjournals.nl: national platform for Diamond Open Access journals.
   

• The Directory of Open Access Journals (DOAJ) contains almost 10.000 OA journals in many languages. More than half of these journals are searchable at article level. 
   

• The Directory of Open Access Scholarly Resources (ROAD) gives access to a subset of the ISSN Register, which describes scholarly resources in Open Access: journals, conference proceedings and academic repositories. ROAD is supported by the Communication and Information Sector of UNESCO.
   

• Scientific Electronic Library Online (SciELO) is an electronic library covering a selected collection of Brazilian scientific journals.
   

• Elektronische Zeitschriftenbibliothek (EZB) provides a world wide overview of (free) e-journals. With a large number of search options.

• Directory of Open Access Books (DOAB) is a discovery service for peer reviewed books published under an Open Access license. DOAB enables you to search the description of the books, not the full-text content, and gives links to the publications full texts at the publisher’s website or repository. It is a service from the OAPEN Foundation.
   

• OAPEN Library contains the full text to many peer reviewed OA books. It enables you to search both the full-text contents and the description of the books. The OAPEN Library and DOAB are both services provided by the OAPEN Foundation.
   

• Google BookSearch is a large, for the most part full text searchable, database with millions of scanned books from libraries or directly from publishers. In advanced search, you can select: search full view only.
   

• Hathi Trust is a partnership of academic & research institutions, offering a collection of millions of titles digitized from libraries around the world. Click 'advanced search' to search by format (book, serial, audio, etc.). Be sure to select "full view only" to find open access resources.