Unpaywall: available for Chrome and Firefox browsers, it is an extension that allows you to search the web for the free version of a contribution for which only the paid version has been identified. Once you have downloaded the extension, when you find an article for which a payment is due, a padlock icon on the right side of the browser will show the existence of versions available for free; by clicking on the padlock it will be possible to directly read the open access version.
OAbutton: available both as a website and as a Firefox browser extension, it allows you to identify the Open Access version of a contribution of which only the paid version is known or, if not available, to forward the request to the author so that they can send you one copy. OAbutton is supported by SPARC (Scholarly Publishing and Academic Resources Coalition), whose goal is to promote open access as a form of dissemination of scientific research.