How in the hell has this thread made it this far without Michael Moore films being mentioned?
Roger & Me (1989) - explores impact of GM plant closing in Moore's hometown of Flint, MI
Bowling for Columbine (2002) - explores connection between gun culture, politics, and media scapegoating/fearmongering
Fahrenheit 9/11 (2004) - highest grossing documentary of all time, explores profiteering around the War on Terror and critiques media complacency in covering lead up to military actions
Sicko (2007) - explores problems in the American healthcare system
Capitalism: A Love Story (2009) - explores the relationship between American capitalism and recent/past economic crises
Moore is certainly a documentary film maker with an agenda for his films, and he provokes a range of fanaticism and vitriol every time he releases a new one. His films at the very least provide the audience with something to think about and research (unless you mindlessly approach documentaries as "objective truth") and there are, to my mind, some very fair critiques across his oeuvre.
In any case, even if you don't care about his subjects or like his ideological bent, Moore is an extremely important documentary film maker in terms of his directorial style. I would guess that many of the directors who produced films named in this thread thus far would cite him as an inspiration for their own efforts and may have studied his films as part of their own training in how to shoot and plan a documentary...