A browser extension (sometimes called an add-on) is piece of software for customizing a web browser, extending its capabilities. There are thousands of extensions available for Chrome, Firefox, Safari and other web browsers, which change the look and feel of the browser or provide extra functionality such as blocking ads, managing passwords or being able to download videos.
There are a few browser extensions for Chrome, Firefox, and Safari which can help you find the full-text of an article.
Unpaywall is a browser extension for Chrome and Firefox that will let you know if there's a free and legal full-text version of a paywalled article. If there is, you'll see a green tab on the right-hand side of the page. Click the green tab to access the full-text
Click the green button to see the full-text
The grey button means that Unpaywall couldn't find the full-text
Unpaywall won't always work, but the creators estimate that you can access the full-text around 50-85% of the time, depending upon the subject area. Unpaywall is the best of these apps and is highly recommended.
Open Access Helper is an app for iOS (Apple) that uses Unpaywall to find full-text. It does a couple of other things as well and is worth looking at if you have an iPhone or Mac
OAButton (formerly Open Access Button) adds a browser toolbar button to click on an article to check to see if there is a full-text version available (as opposed to Unpaywall which will provide access straight away) and will also request the article from the author if it is not available.
The toolbar button
OAButton uses the Unpaywall database and other sources to search for free and legal full-text, so you might have more luck with this than Unpaywall (you could use both and click the OAButton icon if Unpaywall doesn't work)
The academic search engine CORE provides a browser extension that works just like Unpaywall, right down to having a green tab on the right-hand side of the page to click to access full-text. Core uses different sources, including repository data that might be missing from Unpaywall.
Like Open Access Button and Unpaywall, EndNote finds full-text if available. Unlike the other two extensions, EndNote is a product of a commercial publisher (Clarivate) and will also search commercial publishers' sites to which your institution has a subscription, which adds no value to independent researchers or those whose libraries do not have many subscriptions. EndNote Click also requires a login. Here's a comparison of features with Unpaywall
EndNote Click (Kopernio) for Firefox
EndNote Click (Kopernio) for Chrome
Like Unpaywall, Lazy Scholar automatically searches for free full-text. Like Kopernio, it can be configured to search a library's subscriptions. It does a few other things too, such as saving your search history and providing citations in a variety of formats. One issue with Lazy Scholar is that it is the pet project of one developer. If they fall under a bus, or simply get bored with the project, then that's that. It's worth looking at however
This adds a button to your browser toolbar, Highlight text and click the Google Scholar Button to open a box showing a Google Scholar search for the highlighted text.
A problem with Google Scholar - and therefore the button - is that Google keeps secret the criteria for adding material to Google Scholar, and so you might find content that isn't that scholarly at all. Google Scholar doesn't use the same sources as the likes of Unpaywall and Open Access Button but does include content from a couple of sites that may breach copyright.
Google Scholar Button for Firefox
Google Scholar Button for Chrome