You can find an incomplete but worthwhile list here:
http://en.wikipedia.org/wiki/List_of_proven_conspiracies
Contents [hide]
1 1st century BC
1.1 63 BC The second Catiline conspiracy
1.2 44 BC Caesar assassination plot
2 1600s
2.1 1605 The Gunpowder Plot
3 1700's
3.1 1763 Small pox infected blankets given to Native Americans
4 1800
4.1 1810 to 1850 the Underground Railroad
4.2 1865 The Assassination of President Abraham Lincoln
4.3 1894 The Dreyfus affair
4.4 1894 Overthrow of the Kingdom of Hawaii
4.5 1898-1913 Western Water Rights: Los Angeles vs. Owens Valley
5 1900
5.1 1913 Federal Reserve Act
5.2 1919 Black Sox Scandal
5.3 1930's National Crime Syndicate
5.4 1932 General Motors streetcar conspiracy
5.5 1932 to 1972 Tuskegee Syphilis Study
5.6 1945 Nazi gold
5.7 1950 Project MKULTRA
5.8 1953 Operation Midnight Climax
5.9 1953 Operation Ajax
5.10 1954 Operation PBSUCCESS
5.11 1956 COINTELPRO
5.12 1961 Bay of Pigs invasion
5.13 1961 The Cuban Project
5.14 1962 Operation Northwoods
5.15 1964 Gulf of Tonkin incident
5.16 1965 Suppressing Sarkhan
5.17 1967 to ? USS LIBERTY
5.18 1969 Secret Bombing of Cambodia
5.19 1969 to 1982 Italian Strategy of tension
5.20 1970 to ? Air America
5.21 1972 to 1974 Watergate scandal
5.22 1975-1999 Indonesian occupation of East Timor
5.23 1981 to 1986 Iran-Contra
5.24 1988 Nelson Bunker Hunt and Silver Thursday
5.25 1990 Las Vegas American Coin Fixed Video Poker Conspiracy
5.26 1993 Assassination plot against President George H.W. Bush
5.27 1993 WTC terrorist attack
5.28 1994 Diamonds are forever?
5.29 1995 The Bojinka Plot
5.30 1995 Oklahoma City bombing
5.31 1995 to 2004 Jack Abramoff Indian lobbying scandal
6 2000
6.1 2001 Enron and Arthur Andersen
6.2 2002 Las Vegas Venetian Rigged Drawing
6.3 2004 Secret CIA Prisons
6.4 2005 Ahmed Omar Abu Ali
6.5 2005 DRAM price fixing
6.6 2005 Bernard Ebbers