Across the Web today, you may have a hard time accessing your favorite sites.

Try it. Go to BoingBoing and try to read some blog posts. Or try to make out the logos at Reddit or Metafilter.

Those sites and logos are blacked out to draw attention to American Censorship Day, a major effort by a coalition of dozens of groups including Public Knowledge, the Electronic Frontier Foundation, Mozilla and Free Press (longer list here) to kill the “Stop Online Piracy Act.” SOPA (H.R. 3261) is a bill moving through the House of Representatives that could rip apart the fabric of the open Internet — and introduce a new regime of online censorship.

Hollywood and the U.S. Chamber of Commerce are pushing SOPA as a solution to online copyright violations, but its heavy-handed tactics would infringe on the due process rights of the thousands of users who could see their sites disappear from the Internet.

The bill would have dangerous repercussions here in the U.S. and around the world. Global Voices co-founder Rebecca MacKinnon writes today in the New York Times that the bill could actually “strengthen China’s Great Firewall and even bring major features of it to America.”

SOPA would not only let companies silence websites they suspect of posting illegal content but would also allow banks to freeze financial deposits to the accounts of website owners, potentially forcing falsely accused Internet enterprises out of business. Corporations would have way too much authority over the way the Internet works, and the private sector would have the power to disconnect the URLs of any websites corporations contend are behaving improperly.

A Senate version of SOPA, called the Protect IP Act, passed committee approval in the spring following a massive push by film and music industry lobbyists. These lobbyists are back, but now Silicon Valley companies and venture capitalists have joined forces with civil liberties groups, independent musicians and free speech advocates to stop the bill.

American Censorship Day is the fruit of that collaboration. You can get involved, too. Sign our letter against SOPA and use Facebook, Twitter and email to spread the word about this important day of action.