Here's a list of some of the most noteworthy Congressional investigations:
Robert Morris investigation: In 1790, financier Robert Morris asked Congress to investigate his own handling of the country's money during the Revolutionary War in order to clear his name of alleged impropriety. It was the first-ever Congressional probe [source: U.S. House].
John Brown raid: During 1859-60, a Senate select committee investigated the abolitionist firebrand's raid on the arsenal at Harper's Ferry, Va. It was the first-ever Senate investigation [source: U.S. Senate].
Ku Klux Klan: From 1871-72, a Senate committee headed by Sen. John Scott, R-Pa., documented the Klan's terrorist activities in North Carolina and convinced President Ulysses S. Grant to send troops to intervene [sources: Senate, Pearl].
Teapot Dome scandal: A 1923-24 Senate investigation exposed corrupt deals that gave oil companies access to Naval oil reserves in the West, and ultimately led to the bribery conviction of Harding Administration Interior Secretary Albert Fall. Fall was the first cabinet member ever to go to prison [source: Cherny].
Army-McCarthy hearings: In 1954, Sen. Joseph McCarthy, a Republican, investigated alleged Communist influences in the media, the government and the U.S. Army. McCarthy got his comeuppance from Army chief counsel Joseph Welch, who famously asked him: "Have you no sense of decency, sir, at long last?" [source: Politico].
Watergate: A 1973-74 select committee headed by Sen. Sam Ervin probed the burglary at the Democratic National Committee headquarters and other illegal activities carried out by President Richard Nixon's reelection campaign. The probe was one of the forces that eventually drove Nixon to resign from office [source: U.S. Senate].
Intelligence abuses: In 1975-76, a Senate select committee probed abuses of power by U.S. intelligence and law enforcement agencies, including the FBI's COINTELPRO program that tried to disrupt and discredit civil rights figures such as Martin Luther King, Jr. [source: Senate].
Iran-Contra affair: In 1987, a joint Congressional committee put the spotlight on the Reagan Administration's covert sales of missiles to Iran and diversion of the proceeds to aid the Nicaraguan Contra rebels [sources: Senate, Politico].
Big Tobacco: 1994 House hearings led by Rep. Henry Waxman, D-Calif., questioned cigarette industry executives under oath about whether their products were addictive. He later obtained secret documents showing an industry cover-up [source: Greenberg].
Benghazi: Congress held multiple hearings on the cause of the 2012 attack on the Libyan mission and whether the State Department could have prevented it. When former Secretary of State Hillary Clinton first testified, she memorably said, "[T]he fact is we had four dead Americans. Was it because of a protest or was it because of guys out for a walk one night who decided that they'd they go kill some Americans? What difference at this point does it make? It is our job to figure out what happened and do everything we can to prevent it from ever happening again" [source: Politico].