Video documentaries about physics? Does anyone know about good video documentaries about physics, possibly on YouTube?
I've seen a couple of them about string theory, but I could not find many others.  
 A: Feynman's Messenger Lectures are fun. They're available from Microsoft, which requires some special software, and on YouTube.
These are not really documentaries.  IMHO the physics documentaries I've seen (things like PBS's NOVA) are entertainment more than education.  That's not to say they don't have interesting historical value (as opposed to physics pedagogical value), so whether they're worth watching depends on what you want from them.
A: They are probably not on Youtube, but many of BBC's series are pretty good documentaries, although, obviously, quite "popularized".
For example, their 2007 series "Visions of the future" was a 3-part series about "The Intelligence Revolution", "The Biotech revolution" and "The Quantum revolution".
Their Horizon-series also encompasses many popular physics-subjects, perhaps these are what you've seen about string-theory.
I don't remember the name, but one historically interesting documentary was one about how temperature and heat was discovered and how the quest for cooling to yet colder temperatures proceeded. 
On Youtube, the Feynman lectures stand out. Just search for "Feynman lectures" on Youtube, there are many. This is the first of a series of lectures of QED, Quantum Electrodynamics of which he was a pioneer. This lecture is an absolute must-view!
This youtube-user has posted a lot of interviews and talks.
A: You could try Shankar's two lecture courses at Yale, http://oyc.yale.edu/physics. He's pretty highly rated by students at Yale, and there was positive feedback from someone I recommended this to on PhysicsForums. Yale open courses are one of the offerings mentioned on Wikipedia at http://en.wikipedia.org/wiki/OpenCourseWare.
Obviously these are not documentaries as such. They will require more engagement to get something from them. This is not intended to be an Answer in the precise terms of your Question.
A: PBS nova series the elegant universe is informative and thought provoking
http://www.youtube.com/playlist?list=PLmvKWW9SB-_6ExPWrvpfY37ibQCKcqq2D
