Unpaywall
This tool lists the freely accessible versions of scientific articles, whether they are on the publisher's website or in a disciplinary or institutional archive.
You may
- install it as a browser extension (Firefox or Chrome); or
- query it online from DOI(s) of the required article(s)
Once the extension is installed, it displays a padlock-shaped button on the right of the page, when browsing on a publisher's site or in a database. If a freely accessible version is known or detected by Unpaywall, the button turns green.
By clicking on the green button, you are automatically redirected to the pdf of the free version that is hosted on one of the many platforms (directories or Open Access editors) existing throughout the world.
Current coverage: nearly 30 million articles
Read the description of the tool (in French)
More details : http://unpaywall.org/
OTHER EXTENSIONs
Available for Chrome and Firefox
Works in a similar way to Unpaywall.
Once installed, the extension generates a button, this time in the upper right-hand corner of the browser toolbar.
The search for a freely accessible version only starts when you click on the button.
It is also possible to use the tool without downloading the extension, by entering the URL, DOI, PMID,... directly on the OA button website.
- nice little extra: if there is no freely accessible version available, the tool promps you to explain why you are interested in this article and sends a request to the author on your behalf, inviting him to put his article online.
Available for Chrome, Firefox and Safari
Extension that generates a button, in the top right of the browser's toolbar. By clicking on the button, a window with the Google Scholar search results appears.
To get the most out of it, we recommend enabling UNIGE access in the button via the option "Library Links" 
- nice little extra: allows you, as in Google Scholar, to know how many times the article has been cited and helps you create a formatted reference to cite it very easily.
Available for Chrome, Firefox and Opera
Extension that generates a button on the right side of the web page. By clicking on it, you are redirected to the freely accessible version found by the extension.
- nice little extra: if the desired article is not available, the extension recommends other freely accessible articles on related topics
Available for Chrome and Firefox
Free browser extension, but requires you to create an account to download and configure it. With this account, you benefit from a "Locker", or online storage space, in which each PDF you have accessed through the extension is automatically stored.
Generates the display of a button in the bottom left corner of your browser.