The USA has been affected by Stop Online Privacy Act today. Many American websites, as well as the English Wikipedia have been blacked out by their authors and the US Congress themselves.
If this happens, this means half of the world's websites will be blacked out.
Google on its Google.com (USA) site, is trying to allow it's American users to stop censorship on Google.com. However this will not affect, the Google sites in other countries (e.g. Google.co.uk or Google.fr) .
Wikipedia on its English site has been blacked out with a notice. Whilel loading English Wikipedia, it would come up with the article and then with the notice.
However Yahoo! on its American site doesn't add any SOPA/PÎPA notice but it does have news on that bill.
Websites like MicroWiki and Sonic Retro have also been blocked by the SOPA.