Unpaywall
Unpaywall is a project of OurResearch, a nonprofit building tools to help make scholarly research more open, connected, and reusable.
Unpaywall finds the kind of articles you'd see in peer-reviewed scholarly journals like Science or PLOS One, plus pre-publication versions of similar work from preprint repositories like arXiv. Specifically, it looks for articles with a kind of identifier called a DOI. Although the best-known type of paywall is the kind in front of a newspaper or magazine article you want to read, Unpaywall doesn't help with sites like nytimes.com or washingtonpost.com.
Website: Unpaywall