Unpaywall is a service that helps people locate the growing number of open access or free legal versions of journal articles. You may already have seen links to the full-text of journal articles via Unpaywall in the library discovery system, or through the NICE databases for evidence searching.
There is also a really useful Unpaywall browser extension available for Chrome and Firefox (sadly not for Edge) that makes it easy to see if an article is available for free when searching most publisher websites, PubMed, The Cochrane Library, or some other sites. More information about the browser extension is available on the Unpaywall site.
With the browser extension installed, when you're visiting a compatible site, a green unlocked padlock indicates a free version is available, and the padlock can be clicked to take you to the article. A grey padlock indicates no open access, and no padlock means you're on a site or page that does not work with the Unpaywall extension.
There will stil be articles that aren't available via open access, but you may still find you can access the full-text with an NHS OpenAthens account instead if we subscribe to the journal, and failing that, we can order a copy for you so you can avoid paying publisher prices.
Unpaywall looks for any journal articles that are available by one of three different routes:
Gold open access
Articles that are fully open access, perhaps in open access journals, or in hybrid journals. Often, the article's author or their institution has paid an article processing charge to make the article available as open access.
Green open access
Articles available in institutional repositories with the agreement of the publisher, usually as an author manuscript (or postprint). This means it has been peer-reviewed, but may not have been copy-edited or formatted for publication. There may be an embargo on how soon an author manuscript can be made available under green open access,
Bronze open access
Articles where the publisher has allowed free access, but retains the copyright and can withdraw access. A lot of articles on COVID-19 have been made available for free under bronze open access.
If you're curious, it is possible to change the settings in the Unpaywall extension to enable 'Nerd mode' and display different colours for different access routes - gold, green or bronze.