Unpaywall and Core are open collections of over 200 million free scholarly articles which harvest open access content from thousands of publishers, repositories and other data providers and make it easy to find, track and use.
Both have browser plug ins which are downloadable from the links below. Once installed, these will show you if a free open access copy of an article is available by displaying one (or both) company logos. Click on the logo and it will take you to the article. It’s easy and it’s free.