Open access journals are making research findings more accessible within and beyond academia. However, these journals are relatively young and few in number, representing just a drop in the flood of research findings currently being published. As such, publication-quality research in many fields is only available within the pages of traditional fee-for-access journals. These journals frequently allow authors to upload and distribute pre-publication versions of their papers to promote the published versions, and many authors do, but these are often difficult to locate. The Unpaywall browser extension addresses that problem. The extension uses data aggregated from PubMed Central, the DOAJ, Crossref, DataCite, GoogleScholar, and BASE to alert users when a free version of a paper is available. The Unpaywall FAQ estimates that free full text versions can be located from 50-85% of publications depending on topic and publication year. The plugin and the data driving it are provided by Impactstory, a 501(c)(3) non-profit with funding from the National Science Foundation and the Alfred P. Sloan Foundation. Unpaywall is available for Chrome and Firefox.