I've found these documentaries to be very interesting, and they have developed some what of a cult following, particularly among people on the Internet. All three deal with different subjects; some topics include the US monetary system, globalization, social change, conspiracy theories, religion, human nature, natural resources, and economics.
Personally, I do not normally suggest the first film because it has many unsubstantiated conspiracy theories. The second and third film have become two of my favorite documentaries. But, I will let you be the judge. These three films have been released for free on the Internet. The main page for the films is at http://www.zeitgeistmovie.com/.
Zeitgeist: The Movie (2007, 122 minutes)
Zeitgeist: Addendum (2008, 123 minutes)
Zeitgeist: Moving Forward (2011, 161 minutes)