"Dogtown & Z-Boys" - Stacy Peralta's doc on the influential skateboard pack he rode with
PBS has decent ones in their American History; I once watched a great documentary on President Andrew Jackson on A&E Cable in the Classroom.
History Channel. National Geographic. Travel Channel. MTV's The Real World, that shows a lot of medical case documentaries lol
But yeah, there a lot out there
Sharkwater
Probably the most important documentary I've seen in the last few years. A guy sets out on a journey just to find out more about sharks, but it turns into something much more when you realise just how important sharks are to our planet, and how no one seems to notice.
Of course it's not all that simple. Throw in some pirate attacks and a flesh eating disease and you got a winner.