According to Box Office Mojo, Fahrenheit 9/11 is the top-grossing documentary of all time ($119,194,771); followed by March of the Penguins, An Inconvenient Truth, Bowling for Columbine, and Madonna: Truth or Dare.
I'm surprised that The Weather Underground is at #95 and has only made a little over half a million dollars. For a guy who didn't live through the insanity of the sixties and early seventies (me), it seemed a very powerful and thought-provoking film...