Nonprofit altmetrics pioneer Impactstory has launched Unpaywall, a free extension for Google Chrome and Firefox browsers that helps users obtain free full-text copies of open access (OA) research papers. Powered by ImpactStory’s oaDOI index of 90 million OA articles drawn from the Directory of Open Access Journals, CrossRef’s license metadata, DataCite, the BASE OA search engine, institutional repositories (IR), and other sources, the extension displays a green tab on a browser’s border when users are visiting an article download page if an OA copy of the article is available elsewhere. Clicking on the tab then seamlessly redirects users to the OA copy. Impactstory claims that the extension will find full-text OA for 65 percent to 85 percent of articles, depending on topic and year of publication. Unpaywall.org’s FAQ notes that the extension is somewhat similar to the Open Access Button, which also helps facilitate discovery of legally uploaded OA content. Both differ from sites such as Sci-Hub, which has courted controversy by hosting and transmitting non-OA, copyrighted content. Impactstory cofounder Jason Priem wrote in a January blog post that Sci-Hub “gives us a glimpse of what [it] will look like when universal, legal OA becomes a reality.” But he told LJ that the copyright and legal issues surrounding the resource would always present a barrier to its growth and utility. “There's just always going to be a cap on the usefulness of a service like Sci-Hub, because institutional stakeholders can't afford to touch it,” Priem said. “It's hard to do big things and make big changes in that situation. Napster was a big deal, but in the end it didn't change the music industry—iTunes and Spotify did.” Similarly, workarounds such as emailing researchers to ask for a copy of a paper or using #icanhazpdf on Twitter can be “lifesavers when you’re really desperate,” Priem said. “But we think people shouldn't have to be desperate to access a paper, and so we're trying to fix that. We're not done yet, but we're making progress, thanks to the millions of researchers who've made their work free to read online by uploading it to open repositories.” The extension currently includes a simple set of features. Users can choose to have the tab color coded to visually illustrate the type of OA content and its source. If this setting is applied, the standard green tab will indicate content from an institutional repository or preprint server, a gold tab will indicate journal-hosted content with an Open license, and a blue tab will indicate journal-hosted articles with no license. (If a full-text OA copy cannot be found, the extension always displays a gray tab.) Users can also opt to hide content from sources that don’t adhere to the Open Archives Initiatives Protocol for Metadata Harvesting (OAI–PMH) and aren’t registered with BASE. This will exclude results from sources such as ResearchGate, Academia.edu, researcher homepages, and some IRs, decreasing coverage by about 20 percent, a settings menu explains. Priem said that the feature was included because of concerns raised by some within the academic community regarding the policies that for-profit professional networking sites like Academia.edu and ResearchGate have established regarding content access, ownership, and preservation, among other issues. But, he added, “Google Scholar has shown full text results from RG+Academia for years, and that seems to be going fine. We do the same. But we can also appreciate the position of those who'd prefer not to use these sources; hence the setting allowing users to remove them.” Since Unpaywall’s launch in mid-March, it has been adding about a thousand active users per day, Priem told LJ. At press time, almost 40,000 users had downloaded the extension, according to statistics on the Chrome Web Store and Firefox Add-ons. “We have users in 150 countries,” Priem told LJ. “Now, unsurprisingly given its size and position in the global research economy, the U.S. has more Unpaywall users than any other country (30 percent).  But after that, the user base is highly international: More than half of users are in a 'long tail' of 140 countries.” Users in eastern Asia, including China, Japan, Taiwan, and Korea, have accounted for 19 percent of these early downloads, and 31 percent are from Europe. “The key story here is that Unpaywall serves a highly global audience...which is not surprising, since a lot of the world does not enjoy the access we have in the developed world/global north.”