How he blew the whistle: As the associate director of the Federal Bureau of Investigation in the early 1970s, Mark Felt was aware of illegal attempts by the Nixon Administration to spy on political opponents, breaking into offices and reading their mail. So when J. Edgar Hoover died in 1972, and Nixon bypassed Felt to appoint an ally as FBI director, Felt became a critical source to Washington Post reporter Bob Woodward, as he and fellow reporter Carl Bernstein investigated the Watergate Hotel break-in and resulting scandal.
What happened: He retired from the FBI in 1973. For years, Felt's identity was kept secret; Woodward and Bernstein only referred to him as "Deep Throat." At his request, his true identity was disclosed in 2005, when he was 91 years old. He died in 2008.
In 1978, he faced felony charges in connection with FBI surveillance of radical groups such as the Weather Underground. He was pardoned by President Ronald Reagan in 1982. During those legal proceedings, he was the beneficiary of many private donations, including reportedly gifts from Nixon himself. Felt wrote several books about his time in the bureau.
How he blew the whistle: In the 1960s and early 1970s, military leaders and the White House claimed that the U.S. was winning the Vietnam War. In private, however, they wrote an encyclopedic history that detailed America's failures. In 1969, young defense analyst Daniel Ellsberg began photocopying Department of Defense records which documented how four presidential administrations misled the American public. It also revealed the secret bombing campaign in Cambodia. In 1971, Ellsberg handed over the secret war files, known as the "Pentagon Papers," to the New York Times.
What happened: Ellsberg was charged under the Espionage Act of 1917, but the charges were dropped in 1971. Ellsberg began a career as a political activist.
What he's doing now: As a member of the New York-based Campaign for Peace and Democracy, Ellsberg has been an outspoken opponent of the Iraq War, among other issues. He lives in Kensington, Calif., and has called Edward Snowden a "hero."
How he blew the whistle: As a matter of policy, Israel refuses to acknowledge its nuclear program. But as a nuclear technician for the Israeli government, Vanunu confirmed the existence of the program to the Sunday Times of London in 1986, saying that he was morally opposed to such weapons. At the time, Vanunu said that Israel had between 100 to 200 nuclear weapons.
What happened: Israeli agents captured him in Italy in 1986 and Vanunu was sentenced in Israel to 18 years in prison as a traitor -- 11 of them in solitary confinement. After he was released in 2004, Vanunu was sent back to jail in 2010 for three months for meeting with foreigners, including his Norwegian girlfriend.
What he's doing now: Vanunu is still monitored by the Israeli government, and is reported to be living in East Jerusalem.
How he blew the whistle: In 1992, Mark Whitacre, an executive with the pharmaceutical firm Archer Daniels Midland, told his wife that his company was fixing the price of the food additive lysine to increase profits. She pressured him to tell the FBI, according to the 2001 book, "The Informant." Whitacre soon began giving the FBI information on his company, becoming the highest-level corporate whistle-blower. Three years later, however, at ADM's urging the FBI investigated Whitacre and found that he had stolen $9 million from ADM.
What happened: Whitacre was convicted of tax evasion and fraud -- and also was sentenced along with other high-ranking ADM executives for participating in the price-fixing scheme, which ended up costing the company hundreds of millions. He served eight years in prison and was released in 2006. His story was soon made into the 2009 feature film starring Matt Damon, called, "The Informant!"
What he's doing now: He is now the COO of Cypress Systems, a California-based biotechnology firm. He is fighting for a pardon, and many FBI agents support him.
How he blew the whistle: In 1994, the CEOs of the seven major American tobacco companies testified before Congress, swearing under oath that nicotine was not addictive. Soon after Jeffrey Wigand, a former vice president for research and development at the Brown & Williamson tobacco company, sat down for an explosive interview with Mike Wallace of "60 Minutes." Initially, the network didn't broadcast the interview out of fear of breaching Wigand's confidentiality agreement with his employer and the multi-million dollar lawsuit that could ensue. But after the Wall Street Journal reported on Wigand's story CBS decided to air the interview in full. In speaking to Wallace, Wigand said his company knowingly doctored the nicotine content in its cigarettes to enhance its addictive qualities.
What happened: Wigand was eventually sued by his former employer for breaching the confidentiality agreement. But the suit was dismissed by the courts in 1997 as a condition of the federal government's $368 billion settlement with the tobacco industry. After leaving the tobacco company in 1993, Wigand taught chemistry and Japanese in Kentucky public schools, and even won the state's 1996 teacher of the year award. His story was later told in the hit 1999 movie, "The Insider."
What's he doing now: He has since left teaching and now supports tobacco control through his non-profit group SMOKE-FREE KIDS, Inc.
How he blew the whistle: As an independent financial fraud analyst, Markopolos says that it took him just five minutes to figure out Bernard Madoff's security fund was a fraud. He said the guaranteed rates of return between 12 to 20 percent were simply impossible. And so starting in 2000 Markopolos began warning the Securities and Exchange Commission of Madoff's Ponzi scheme. In total, he submitted five written warnings to the SEC.
What happened: The fraud only stopped when Madoff said that he couldn't meet the redemption requests in 2008. At that point, the fraud had grown to $65 billion. Markopolos thinks that it was only $5 billion when he first warned the SEC.
What he is doing now: After the Madoff scandal broke, Markopolos received much adulatory media coverage, and the private fraud investigator said that he found himself "under siege" with professional offers, according to the The Boston Globe. In 2011, he also published a book about his Madoff experience, No One Would Would Listen: A True Financial Thriller.
How she blew the whistle: In August 2001, Sherron Watkins, then Enron's vice president for corporate development, sent an explosive, seven-page email to the energy company's CEO, Kenneth Lay. She detailed what she called an "elaborate accounting hoax," which included inflating income and hiding epic losses. Although Lay claimed that he'd launch an investigation, Watkins said that she was immediately punished; her computer's hard drive was confiscated and her desk relocated to the nether regions of Enron. Four months later, Enron could no longer sustain the fraud, and it filed for bankruptcy.
What happened: Five months after she wrote the memo, Watkins was called to testify before Congress and faced criticism for not having notified authorities. By the end of 2002, though, Enron was bankrupt, and Time named Watkins one of the three "People of the Year," and she got a book deal.
What she is doing now: She has since become a public speaker on leadership lessons and problems in U.S. corporate culture. She still lives in Houston but told CNBC that she couldn't continue in business because the label "whistle-blower" made her "radioactive."
How she blew the whistle: As the head of internal auditing for telecommunications giant WorldCom, Cynthia Copper noticed irregular accounting practices in 2002, but the company's CFO brushed her off. She and her team investigated and uncovered a scheme in which WorldCom had been manipulating its operational costs and reporting them as capital expenditures. When she confronted the controller, he confessed, and WorldCom publicly acknowledged its $3.8 billion fraud and filed for bankruptcy.
What happened: Congress, outraged, passed financial reform legislation, the Sarbanes-Oxley Act. Cooper was named a 2002 Time magazine person of the year (along with Enron's Sherron Watkins). WorldCom's CFO pleaded guilty and got five years in prison; and WorldCom CEO Bernie Ebbers was convicted and sentenced to 25 years in prison.
What she's doing now: In 2008, she wrote a book, Extraordinary Circumstances, about her WorldCom experience. She now runs her own consulting firm in Mississippi and speaks of her experience to corporations and universities around the country.
How he blew the whistle: In the aftermath of the Sept. 11 attacks, the National Security Agency began a secret surveillance program to avert future terrorist stikes. The NSA's counterterrorism program allowed the agency to analyze data shared over cell phones and e-mail. A top executive at the NSA, Drake felt the program was unlawful and unnecessary and he leaked top-secret documents about it to Baltimore Sun reporter Siobhan Gorman in 2006.
What happened: The federal government charged Drake for violating the Espionage Act, and he faced a potential 35-year prison sentence. But eventually the feds dropped the felony charges and, in June 2011, Drake pleaded guilty to a misdemeanor, "exceeding the authorized use of a computer." He never spent a day in jail, and was absolved of mishandling classified information.
What he's doing now: Drake works in retail at an Apple store in suburban Maryland. Attorney General Eric Holder was even one of his customers, according to the The Wall Street Journal. He also regularly makes appearances in the media to discuss national security issues, and he's done a flurry of interviews concerning Edward Snowden, warning that the government will "seek revenge and retaliation."