Unpaywall is a service that indexes open access repositories,
university, government and scholarly society archives, and other sources
that make articles available with authorization from the rightsholders
and journals – about 47% of the articles that its users seek.
It runs as a browser extension that adds notifier to your browser window
when you load a scholarly article, with an icon that tells you whether
there’s an open access fulltext, and whether it’s “green” or “gold” OA.
