1. Fahrenheit 9/11 (2004)
Michael Moore's tirade against the Bush administration's war on terror ignited arguments over everything from the Patriot Act to The Pet Goat. Along the way, it grossed a record $222 million worldwide.
2. Super Size Me (2004)
Filmmaker Morgan Spurlock ate only McDonald's for a month and gained 25 lbs. The fast-food chain has since canceled the Super Size option and added nutritional info to its packaging. Coincidence?
3. An Inconvenient Truth (2006)
Davis Guggenheim's Oscar-winning doc relied on Al Gore's surprising showmanship to raise public awareness about climate change. The film is now viewed in classrooms around the globe.
4. For the Bible Tells Me So (2007)
Detailing how Scripture is interpreted to justify discrimination against homosexuals, Daniel Karslake's film has become a godsend for gay youths coming out to their religious families.
5. The Cove (2009)
With the aid of hidden cameras, Louie Psihoyos' riveting doc (an Oscar winner this year) uncovered the annual slaughter of dolphins in Taiji, Japan.
6. The Thin Blue Line (1988)
Errol Morris' groundbreaking film about Randall Dale Adams, a man who was sentenced to life in prison for allegedly murdering a Dallas police officer, ultimately resulted in Adams' conviction being overturned. Through a multitude of interviews and innovative crime-scene reenactments, Morris revealed that many of the witnesses had lied under oath.
7. Harlan County, USA (1976)
Barbara Kopple's Oscar-winning documentary depicted a group of coal miners' prolonged and frequently dangerous strike against the Brookside Mine in Harlan County, Ky. In addition to raising awareness of the miners' fight for safer working conditions and appropriate wages, Kopple likely prevented an outburst of violence simply by having her cameras on the scene.
8. Triumph of the Will (1935)
Leni Riefenstahl's ''documentary'' — cinema's most notorious (and influential) propaganda film — presented the 1934 Nazi Party Congress in Nuremberg so slickly and skillfully that it helped solidify Adolf Hitler's control over the German citizenry. Frank Capra famously said the following regarding Triumph: ''[It] fired no gun, dropped no bombs, but as a psychological weapon aimed at destroying the will to resist, it was just as lethal.''
9. Why We Fight (1942-45)
The United States' Office of War Information commissioned this series of seven propaganda films from Hollywood filmmaker Frank Capra, who frequently used and recontextualized clips from German propaganda (including the next film in this gallery, Triumph of the Will) to convey the necessity of America's involvement in World War II. The first film in the series, the Oscar-winning Prelude to War, was shown to U.S. troops before they were deployed.
10. The Times of Harvey Milk (1984)
Rob Epstein's Oscar-winning film chronicled the late San Francisco supervisor Harvey Milk's ascent into politics and his trailblazing impact as a gay rights activist. The documentary was essentially retold in the 2008 biopic Milk.
11. Man with a Movie Camera (1929)
This Soviet Union silent film is on this list not for any kind of political influence, but rather for its exuberant exploration of cinematic techniques. Director Dziga Vertov and his camera captured the daily city life of Odessa, Ukraine, and then constructed a dizzying narrative that featured split screens, double exposure, fast motion, freeze frames, jump cuts, and stop-motion animation. No movie has ever seemed quite as energized by the possibilities of the medium.
12. The Up Series (1964, 1970, 1977, 1984, 1991, 1998, 2005)
Influential for its epic scope, The Up Series has followed the same group of British children every seven years since 1964. The novel idea has been repeated in numerous other countries, and director Michael Apted is expected to start shooting the next entry in the series, 56 Up, by 2012.