Earlier today, January 18th, we've announced that the openSUSE and Fedora projects decided to protest against SOPA (Stop Online Piracy Act) and PIPA (PROTECT IP Act) through their websites.
In the mean time, more Internet companies join the strike, such as The Document Foundation with their LibreOffice website (still allowing access to the rest of the site) and the XMBC project's website, saying "Today we are proud to stand together with organizations around the world to protest SOPA and its Senate counterpart PIPA [...] All services will be restored on January 19th, EST time. In the meantime, we urge our users to get involved."
As you probably know already, SOPA (Stop Online Piracy Act) is a bill introduced in the United States House of Representatives.
If made law, it will enable copyright holders and U.S. law enforcement to fight online trafficking in counterfeit goods and copyrighted intellectual property.