Unpaywall

Browser (Chrome/Firefox) extension that uses the DOI (Digital Object Identifier) to help you find contents and versions of open-access scientific and academic documents. The application alerts you with a green padlock whenever it detects the open-access version of a paid-for paper. It also provides access to its database of open-access content from over 50,000 publishers and repositories. 

Study area
Subject:  Multidisciplinary
What can you find?
Subject:  Browser extension
Who can access this?

Open Access