A few websites noted off hand will be participating in a blackout day (January 18, 2012), to publicly oppose the Stop Online Piracy Act. Websites include Reddit, Wikipedia, Imgur, and possibly others. If you know any, feel free to leave them in the comments. Journal Five will also be participating in the blackout.
Opposing SOPA is everyone's best interest. There's no reason a company should be able to single handedly, without court procedure shut down a website, or any portion of a website, or disconnect a user's privileges of a website for the purpose of self-interest. While there could be people flying around posting Copyright materials, it's best if the DMCA is used.
Stop Online Piracy Act simply complicates this, or more-so overlaps it. It's no one's fault the DMCA may be ignored, but from my understanding the majority of the time it is not, and those that receive it follow it accordingly.
YouTube, being arguably one of the most active and used video platforms on the web is at major stake for such a law. Even though YouTube may have the technology to monitor videos, review reports, etc, this does not mean they can do it exactly when a video is uploaded, and respond within ten minutes to every takedown notice.
To the more serious concerns of these bills, news sites may be at risk for their reporting. The way I see it, is the news agency will be at risk for losing their website for the material they report, because someone may not want something published -- Contradiction? That's what news reporting is supposed to be--following behind ethics.
further (this is a detailed explanation. I encourage you to read it) :