Beginning at 05:00 UTC on Wednesday, January 18, the English-language Wikipedia will be blacked out for 24 hours. The blackout is a protest against proposed legislation in the United States — the Stop Online Piracy Act (SOPA) in the U.S. House of Representatives, and the PROTECT IP Act (PIPA) in the U.S. Senate — that, if passed, would seriously damage the free and open Internet, including Wikipedia.
This pending US legislation would make it impossible to run any website that links or allows commenters to link, by making them liable for copyright infringement on the sites they link to.
Mozilla, reddit, the Cheezburger Network and Boing Boing are some of the others joining in the protest.
tip: If you really, really, really need to read a Wikipedia article during these 24 hours: use a mobile app (like Wapedia) or go to http://simple.wikipedia.org/