Wikipedia plans to take its English-language site offline on Wednesday as part of protests against proposed anti-piracy laws in the US.
The user-generated news site Reddit and the blog Boing Boing have also said they will take part in the "blackout".
The sites' webmasters are opposed to the Stop Online Piracy Act (Sopa) and Protect Intellectual Property Act (Pipa) being debated by Congress.
However, Twitter has declined to take part in the shutdown.
Wikipedia's founder, Jimmy Wales, told the BBC: "Proponents of Sopa have characterised the opposition as being people who want to enable piracy or defend piracy.
"But that's not really the point. The point is the bill is so over broad and so badly written that it's going to impact all kinds of things that, you know, don't have anything to do with stopping piracy."
Sopa's supporters in the House of Representatives say the legislation is designed to stop revenue flowing to "rogue websites". A similar bill, Pipa, is making its way through the US Senate.
On Saturday the White House issued a statement that appeared to side with critics of the legislation.
It said: "While we believe that online piracy by foreign websites is a serious problem that requires a serious legislative response, we will not support legislation that reduces freedom of expression, increases cybersecurity risk, or undermines the dynamic, innovative global internet."
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Sopa and Pipa explained
The US bills are designed to block access to sites containing unauthorised copryight material.
Content owners and the US government would be given the power to request court orders to shut down sites associated with piracy.
Advertisers, payment processors and internet service providers would be forbidden from doing business with infringers based overseas.
Sopa also requires search engines to remove foreign infringing sites from their results, a provision absent in Pipa.
Despite the hint of a presidential veto, Wikipedia said that the English site's administrators had decided to stage its first ever public protest because the bills "would be devastating to the free and open web".
It added: "We don't think Sopa is going away, and Pipa is still quite active. Moreover, Sopa and Pipa are just indicators of a much broader problem. All around the world, we're seeing the development of legislation intended to fight online piracy, and regulate the internet in other ways, that hurt online freedoms."
However, when asked whether Twitter would join the blackout, its chief executive, Dick Costolo, tweeted: "Closing a global business in reaction to single-issue national politics is foolish."
In a Twitter conversation with Wikipedia's founder Jimmy Wales, Mr Costolo later clarified that his comment was not meant to be read as a "value judgement" about other organisations involvement in the action.
The anti-piracy legislation still has high profile supporters including News Corporation's chairman, Rupert Murdoch.
A message by Twitter's chief executive sparked off a conversation with Wikipedia's founder
Over the weekend he tweeted: "So Obama has thrown in his lot with Silicon Valley paymasters who threaten all software creators with piracy, plain thievery."
He later added: "Seems like universal anger with POTUS [President of the United States] from all sorts of normal supporters... Whole entertainment industry employs 2.2 million [on] average salary $65,000. Good jobs and expanding foreign earnings. Made in America, too!"
Sites taking part in the shutdown plan to go offline for 24 hours from midnight Eastern Standard Time (05:00 GMT) on Wednesday