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:
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 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.
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.