Unpaywall is a free browser extension for Firefox and Chrome, as well as a database of open access articles. When you come up against an article that is behind a paywall, Unpaywall searches its database for a free to read version for you, and produces a button on the right side of your screen, which takes you to the free version.
Unpaywall searches journals and repositories, so it is completely legal. It does not harvest any sources where the legality of uploads could be questionable (e.g. ResearchGate.) True open access aficionados can even turn on ‘OA Nerd Mode’ which colour codes the button to show whether the free version is gold, green, or bronze open access!
Add the browser extension to start locating free articles.