Another one which seems to go under the radar / wasn't as successful as it should have been is Space Tourists (2009), its a Sundance winner but I literally never meet anybody online who has ever seen it!
Stumbled across this forum via Reddit after finding out that the IMDB forums had closed down, WTF! I didn't post there much, I was more of a lurker.
My all time favourite documentary is an old HBO classic Black Tar Heroin: The Dark End of the Street (link goes to YouTube). Its about the heroin...