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The plan is simple: Sites participating in the protest will go dark for the day or take some other action. Wikipedia, for example, will black out the English-language portions of its site for 24 hours. The move will likely shut out some 10 million users during the course of the day.

Politico pegs the estimated number of sites that will be affected in some way by the protest at 7,000.

Among the sites participating:

• Google will post a link on its home page to a document explaining its opposition to the bill.
• Mozilla.com, home of the popular Web browser Firefox, will go dark will do two things, see the update below.
• Reddit, the social news site owned by Advance Publications, will go dark.
• WordPress.org will go dark.
• TwitPic, the popular site where Twitter users share photographs, will go dark.
• MoveOn.org, the liberal-leaning political site, will go dark.
• The Cheezburger network, including sites like The Daily What and Fail Blog, will be dark.
• BoingBoing.net will be dark.
• Several gaming companies, including Minecraft.net, Riot Games, Epic Games, 38 Studios and Red 5 Studios, will be dark.

Update: Mozilla just sent a statement outlining what it will do for the protest: It will redirect traffic from the main Mozilla.org and Mozilla.com English websites to an action page for 12 hours on Wednesday, January 18th from 8 AM to 8 PM Eastern Time. It will also make the default Firefox start page black so that the tens of millions of Firefox users will see a black page with a call to action message rather than the traditional white page with the Firefox logo.

Since the list is in flux, Irish bookmakers saw a chance to get into the act by accepting bets concerning which sites will go down for the day and which ones won’t.

Wikileaks was the favorite at 5-to-1 odds that it would join the protest. Myspace, the once mighty social network, was running a close second at 7 to 1, while Flickr, the Yahoo-owned photo sharing site, was at 8 to 1. Here’s a list of additional bets that Paddy Power was accepting:

14/1 YouTube
40/1 Amazon
50/1 Yahoo!
66/1 Facebook
66/1 TMZ
66/1 IMDb
80/1 LinkedIn
80/1 EMI
100/1 Twitter
100/1 eBay
100/1 AOL
100/1 iTunes
100/1 HBO
100/1 MSN
200/1 Sony
200/1 Universal Studios
200/1 Bing
200/1 Ask
250/1 BBC
250/1 Disney
500/1 Google
500/1 Fox