Unpaywall
unpaywall.org
Free browser extension and database that finds legal, open-access versions of paywalled research papers.
Why it is useful
Unpaywall scans over 50,000 publishers and repositories to find free, legal copies of paywalled papers (often the author's own accepted manuscript hosted legitimately), surfacing them automatically as you browse. It's built and maintained by a nonprofit and used as a data source by Google Scholar and other academic tools.