Unpaywall consists of a database that includes a list of almost 20 million freely available scholarly articles. Most researchers access it using a browser plug-in that was released in 2017.
Impactstory also offers free access to the Unpaywall database (updated twice a year for non-subscribers).
Unpaywall’s technology has also been integrated into many university-library discovery systems like Ebsco Discovery Service - SISSA Discovery Service.
"This free service locates open-access articles and presents paywalled papers that have been legally archived and are freely available on other websites to users who might otherwise have hit a paywalled version. Since one part of the technology was released in 2016, it has become indispensable for many researchers. And firms that run established scientific search engines are starting to take advantage of Unpaywall".
"On 26 July, Elsevier announced plans to integrate Unpaywall into its Scopus database searches, allowing it to deliver millions more free-to-read papers to users than it does currently. Scopus’s embrace of Unpaywall, along with similar moves by other search engines, means that much more open-access content is now at researchers’ fingertips. These deals are also enabling funders, librarians and others to study open-access publishing trends comprehensively for the first time".
"In June 2017, Unpaywall was integrated into a popular science search engine called Web of Science, which is operated by Clarivate Analytics".
(cit. from "Nature", August 15, 2018, "How Unpaywall is transforming open science").