Unpaywall is an open database in which around 32 million free-of-charge accessible texts are collected. With the help of a browser extension for Chrome, it offers simple and legal access to the Open Access texts for download.
Unpaywall traces the Open Access texts on the basis of the DOI. The data, which serve to detect the Open Access texts, originate from Crossref and the Directory of Open Access Journals (DOAJ). The majority of the data however comes from the fact that Unpaywall itself observes more than 50,000 storage locations on the Web. These include gold OA journals, hybrid journals, institutional repositories and subject-specific repositories.