There are a number of browser extensions or plug-ins for Chrome, Firefox, and Safari which can be installed to facilitate finding articles which are open access:
Unpaywall
Unpaywall
makes finding OA-articles easy for the individual user by installing a plug-in on Chrome or Firefox. Content from Unpaywall is also integrated into other services such as databases and link-resolvers.
Open Access Button
You can search Open Access Button directly on their website or download an extension for Chrome which makes finding open access articles easy. They also offer integrated services for libraries, as interlibrary loans (ILL/DDS). When OA Button hits a paywall, the service also sends off requests to authors asking them to deposit their articles in a subject or institutional repository in order to make their research open. Open Access Button is a non-profit organisation.
Google Scholar Button
Google Scholar Button is a browser extension available for Firefox, Chrome, and Safari. The extension makes finding full-text open access articles in Google Scholar easier and allows access to closed articles which a user has access to via their own institution’s subscriptions, by configuring Google Scholar with your library’s URL.
Lazy Scholar
The Lazy Scholar extension finds open access articles and provides metrics and citations. Additionally, it can include articles behind paywalls via a user’s own institution’s subscriptions.
Kopernio
Kopernio is an extension finding both open access versions of articles and articles which users have access to via institutional subscriptions. The service is free but belongs to Clarivate Analytics and you need to register in order to use the extension.