Tool: Unpaywall | Netbib Weblog

Google English: "When researching one comes again and again to the point that one encounters journal articles or essays in compilations, which are subject to a charge. With the help of a given DOI (digital objects identifyer), several comparable services that can be inserted into the browser as an extension or add-on check whether it can find free versions of the searched text (for example about the secondary publishing rights of the authors)...."