Unpaywall finds free versions of paywalled papers : Nature News & Comment

"An online widget that trawls the Internet searching for free-to-read versions of paywalled papers has been installed more than 10,000 times since its prerelease debut on 10 March, its inventors say. 

Officially launched on 4 April, Unpaywall is a free web-browser extension that hunts for papers in more than 5,300 repositories worldwide, including preprint servers and institutional databases. Once installed in Google Chrome or Mozilla Firefox, Unpaywall brings up a green or grey tab on the side of the screen when a user hits a paywalled paper. A green ‘unlock’ sign means that a free version of the paper is available elsewhere, whereas a grey ‘lock’ icon means that the tool could not locate a free version. For researchers and librarians, Unpaywall and a growing collection of similar tools promise to simplify the process of finding freely accessible (and legal) copies of research articles."