Is there any tool to find free versions of paywalled articles?

Yes. ImpactStory has created a tool called Unpaywall that is based on data from the oaDOI project. Unpaywall indexes over 10 million articles. Researchers now have access to open access full-text versions of paywalled articles. Unpaywall is a web browser extension that finds free versions of paywalled papers and is completely free and legal.

Unpaywall can be quickly installed in Google Chrome or Mozilla Firefox browsers. Once installed, it searches more than ten million uploaded open access resources using an algorithm that compares article titles. A gray lock symbol indicates that the search was unsuccessful and an open access version of that paper is not available. A green unlock symbol flags paywalled research articles with open access versions available elsewhere. Simply clicking on the green unlock symbol next to a paywalled article allows you to read it for free automatically.

Unpaywall represents a more user-friendly version of similar tools that were made available earlier. Impactstory, the team behind Unpaywall, had already developed oaDOI, which retrieves open access articles from a number of sources. The user simply inserts the article’s digital object identifier (DOI) into a search box and any available open access version is retrieved. The main advantage of Unpaywall is that it works in the background, automatically searching for and flagging open access articles while you browse. This makes searching the literature far less tedious and time-consuming.