They translate writings and sermons once largely out of reach of English readers and often feature charismatic clerics like Anwar al-Awlaki, who exchanged dozens of e-mails with the Army psychiatrist accused of the Fort Hood shootings.
The U.S.-born al-Awlaki has been an inspiration to several militants arrested in the United States and Canada in recent years, with his Web-based sermons often turning up on their computers.
"The point is you don't have to be an official part of Al Qaeda to spread hatred and sectarian views," said Evan Kohlmann, a senior investigator for the New York-based NEFA Foundation, which researches Islamic militants.
"If you look at the most influential documents in terms of homegrown terrorism cases, it's not training manuals on building bombs," Kohlmann said. "The most influential documents are the ones that are written by theological advisers, some of whom are not even official Al Qaeda members."
Most of the radical Islamic sites are not run or directed by Al Qaeda, but they provide a powerful tool for recruiting sympathizers to its cause of jihad, or holy war, against the United States, experts who track the activity said.
The number of English-language sites sympathetic to Al Qaeda has risen from about 30 seven years ago to more than 200 recently, said Abdulmanam Almushawah, head of a Saudi government program called Assakeena, which works to combat militant Islamic Web sites.
In contrast, Arabic-language radical sites have dropped to around 50, down from 1,000 seven years ago, because of efforts by governments around the world to shut them down, he said.
Al Qaeda has long tried to reach a Western audience. Videotaped messages from its leader, Usama bin Laden, and his deputy Ayman al-Zawahri usually have English subtitles.
But translations of writings and sermons that form the theological grounding for Al Qaeda's ideology, along with preachers like al-Awlaki, mostly eliminate the language barrier.