With the Unpaywall extension when you see the green tab beside a research article, click it to read the full text. You'll see our green tab on about half of articles (if we can't find fulltext, you'll see a gray tab ). Happy reading!
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Research User Guide
PS for the Open Access nerds: the Unpaywall tab can change colors to indicate Green or Gold OA, too. Enable it in settings. You might also enjoy our recent paper analyzing trends in OA and the coverage of Unpaywall.