Probably the best documentary ive seen was "The Cove" its about Japans slaugther of doplhins and theyre mercury poisoning; its won an oscar. ( I didnt use to give a shit about no dolphins ).
Another very interesting documentary is Ancient Aliens wich has aired on the history channel. Even if you dont believe in aliens you will see great monolithic structures and other mysteries of the earth among other things; you can find it in HD in youtube. I really recommend for you to see both of these documentaries
EDIT: If you wanna watch some drug related documentary Reindeerspotting was quite good, its about a finnish junkies life.
Another very interesting documentary is Ancient Aliens wich has aired on the history channel. Even if you dont believe in aliens you will see great monolithic structures and other mysteries of the earth among other things; you can find it in HD in youtube. I really recommend for you to see both of these documentaries

EDIT: If you wanna watch some drug related documentary Reindeerspotting was quite good, its about a finnish junkies life.