From April 4th 2017 there is a free tool available as a firefox or google chrome browser add-on which automatically searches for legal open access versions of research articles behind a paywall. Here is a link (you can install the pre-release versions right now):
This is fully legal and differs in that respect from Sci-Hub.
Firstly I need to qualify my remark that Sci-Hub is illegal. In Switzerland it is not illegal to download papers from Sci-Hub.
Also, there is a little user guide to Unpaywall and another tool Open Access Button. It was sent to me from another unifr-library and I have adapted it with examples from philosophy: