These online search tools harvest Open Access content from repositories around the world allowing you to search huge databases of content from one simple search box.
If you run into a paywall and can't access a particular article, you can also use these online platforms to search for Open Access versions of your article. Just use the bibliographic information to search for the article.
Bielefeld Academic Search Engine, operated by Bielefeld University Library, is one of the world's most voluminous search engines especially for academic web resources. BASE provides more than 150 million items from more than 7,000 sources. About 60% of the indexed documents are OA.
Unpaywall harvests Open Access content from 50,000 publishers and repositories creating a database with access to over twenty million Open Access articles.
If researchers hit a paywall the Unpaywall browser extension searches this extensive database and notifies you if it locates freely accessible full-text versions of the article.
The Open Access Button searches across the web for a freely accessible full-text version of any article.
You can search for articles on the Open Access Button website or download the browser extension.
COnnecting REpositories (CORE)
CORE is a service from the Open University, UK. It aims to aggregate all of the world's Open Access content and provides researchers with access to over 130 million research papers through it's searchable database.