A free, multilingual encyclopaedia powered by user contributions. It was started in January 2001 by Jimmy Wales and Larry Sanger and has since rose to become the 6th largest site on the internet, housing over 20 million articles in 240 languages.

On January 18th 2012, wikipedia will take down the English version of its website in order to protest against the Stop Online Piracy Act (SOPA), and the closely related Protect Intellectual Property Act (PIPA). These acts both directly threaten the continued existence of wikipedia, so the blackout is designed to show web users what life will be like without the website.