Sci-Hub provides free access to millions of research papers that are otherwise locked behind publisher paywalls. Founded in 2011 by Alexandra Elbakyan, it works by retrieving papers through institutional access and then serving them to anyone who enters a DOI or URL. The site currently holds over 88 million documents, covering the vast majority of published research across science and medicine.

It's widely used by researchers in countries where institutional subscriptions are unaffordable, and its existence has sparked ongoing debates about copyright, open access, and the economics of academic publishing. Legally, it operates in a gray area in most jurisdictions and has been blocked in several countries. Users should be aware of the legal situation in their own location before using it.