Former intelligence worker Edward Snowden revealed himself as the source of documents outlining a massive effort by the NSA to track cell phone calls and monitor the e-mail and Internet traffic of virtually all Americans. He says he just wanted the public to know what the government was doing. "Even if you're not doing anything wrong, you're being watched and recorded," he said. Snowden has been granted temporary asylum in Russia after initially fleeing to Hong Kong. He has been charged with three felony counts, including violations of the U.S. Espionage Act, over the leaks.

Military analyst Daniel Ellsberg leaked the 7,000-page Pentagon Papers in 1971. The top-secret documents revealed that senior American leaders, including three presidents, knew the Vietnam War was an unwinnable, tragic quagmire. Further, they showed that the government had lied to Congress and the public about the progress of the war. Ellsberg surrendered to authorities and was charged as a spy. During his trial, the court learned that President Richard Nixon's administration had embarked on a campaign to discredit Ellsberg, illegally wiretapping him and breaking into his psychiatrist's office. All charges against him were dropped. Since then he has lived a relatively quiet life as a respected author and lecturer.

Starting in 1932, the U.S. Public Health Service studied untreated syphilis in black men who thought they were getting free health care. The patients weren't told of their affliction or sufficiently treated. Peter Buxtun, who worked for the Public Health Service, relayed information about the Tuskegee syphilis experiment to a reporter in 1972, which halted the 40-year study. His testimony at congressional hearings led to an overhaul of the Health, Education and Welfare rules concerning work with human subjects. A class-action lawsuit was settled out-of-court for $10 million, with the U.S. government promising free medical care to survivors and their families. Here, participants talk with a study coordinator.

In 2005, retired deputy FBI director Mark Felt revealed himself to be the whistle-blower "Deep Throat" in the Watergate scandal. He anonymously assisted Washington Post reporters Carl Bernstein and Bob Woodward with many of their stories about the Nixon administration's cover-up after the June 1972 break-in at the Democratic National Committee headquarters. The stories sparked a congressional investigation that eventually led to President Nixon's resignation in 1974. The Post won a Pulitzer Prize for its coverage. Felt was convicted on unrelated conspiracy charges in 1980 and eventually pardoned by President Ronald Reagan before slipping into obscurity for the next quarter-century. He died in 2008 at age 95.

President Ronald Reagan addresses the media in 1987, months after the disclosure of the Iran-Contra affair . A secret operation carried out by an American military officer used proceeds from weapons sales to Iran to fund the anti-communist Contras in Nicaragua and attempted to secure the release of U.S. hostages held by Iran-backed Hezbollah in Lebanon. Mehdi Hashemi, an officer of Iran's Islamic Revolutionary Guards Corps, leaked evidence of the deal to a Lebanese newspaper in 1986. Reagan's closest aides maintain he did not fully know, and only reluctantly came to accept, the circumstances of the operation.

Tobacco industry executive Jeffrey Wigand issued a memo to his company in 1992 about his concerns regarding tobacco additives. He was fired in March 1993 and subsequently contacted by "60 Minutes" and persuaded to tell his story on CBS. He claimed that Brown & Williamson knowingly used additives that were carcinogenic and addictive and spent millions covering it up. He also testified in a landmark case in Mississippi that resulted in a $246 billion settlement from the tobacco industry. Wigand has received public recognition for his actions and continues to crusade against Big Tobacco. He was portrayed by Russell Crowe in the 1999 film " The Insider ."

For 10 years, Frederic Whitehurst complained mostly in vain about practices at the FBI's world-renowned crime lab, where he worked. His efforts eventually led to a 1997 investigation that found lab agents produced inaccurate and scientifically flawed testimony in major cases, including the Oklahoma City and World Trade Center bombings. The Justice Department recommended major reforms but also criticized Whitehurst for "overstated and incendiary" allegations. He also faced disciplinary action for refusing to cooperate with an investigation into how some of his allegations were leaked to a magazine. After a yearlong paid suspension he left the bureau in 1998 with a settlement worth more than $1.16 million.

FBI whistle-blower Coleen Rowley accused the bureau of hindering efforts to investigate a suspected terrorist that could have disrupted plans for the September 11, 2001, terror attacks. In 2002 she fired off a 13-page letter to FBI Director Robert Mueller and flew to Washington to hand-deliver copies to two members of the Senate Intelligence Committee and meet with committee staffers. The letter accused the bureau of deliberately undermining requests to look into Zacarias Moussaoui , the only person convicted in the United States of playing a role in the attacks. She testified in front of Congress and the 9/11 Commission about the FBI's mishandling of information. Rowley was selected as one of Time magazine's People of the Year in 2002 , along with whistle-blowers Sherron Watkins of Enron and Cynthia Cooper of WorldCom.

Sherron Watkins, a former vice president at Enron, sent an anonymous letter to founder Kenneth Lay in 2001 warning him the company had accounting irregularities. The memo eventually reached the public and she later testified before Congress about her concerns and the company's wrongdoings. More than 4,000 Enron employees lost their jobs, and many also lost their life savings, when the energy giant declared bankruptcy in 2001. Investors lost billions of dollars. An investigation in 2002 found that Enron executives reaped millions of dollars from off-the-books partnerships and violated basic rules of accounting and ethics. Many were sentenced to prison for their roles in the Enron scandal

In 2003, federal air marshal Robert MacLean anonymously tipped off an MSNBC reporter that because of budget concerns, the TSA was temporarily suspending missions that would require marshals to stay in hotels just days after they were briefed about a new "potential plot" to hijack U.S. airliners. The news caused an immediate uproar on Capitol Hill and the TSA retreated, withdrawing the scheduling cuts before they went into effect. MacLean was later investigated and fired for the unauthorized disclosure of "sensitive security information."

Photos: Notable leakers and whistle-blowers

Army Pfc. Bradley Manning was convicted July 30 of stealing and disseminating 750,000 pages of classified documents and videos to WikiLeaks, and the counts against him included violations of the Espionage Act. He was found guilty of 20 of the 22 charges but acquitted of the most serious charge -- aiding the enemy. Manning is set to speak in his defense when he takes the stand during the sentencing phase of his court-martial on Wednesday, August 14. He could face up to 90 years in prison if the judge imposes the maximum sentence.