Today has been chosen as a day of protest for many websites who wish to protest against two bills being considered by the US, Stop Online Piracy Act (SOPA) and PROTECT IP Act (PIPA). These two acts would give authorities the power to seize domains without due process, which could effectively lead to sites being blacked out. To show their support, many sites are also blacking out to show how things may be under SOPA/PIPA. The highest profile site to do this is Wikipedia:
Other sites taking part include Reddit and imgur. Google also have a doodle on their US site showing the logo blacked out:
Another site showing their support is TheOatmeal, as suggested I am pirating their gif!
Hopefully these protests will lead the US authorities to realise that their proposed acts will be a big mistake and could cause major damage to the Internet as we know it.