Last school year I used Discovery Ed a lot because, besides teaching 6th grade language arts and newcomer language arts, I taught newcomer social studies and science. I found Discover Ed to be useful for visual purposes. In my regular language arts classes, I used YouTube videos to enhance specific curriculum topics such as dialect and tools used in advertising.
For our persuasive unit, I always use a video made by Bruce Goodner to explain different ways people try to persuade someone. See below. Last year I used a video of top 10 commercials and had the students analyze them for effectiveness.
With regard to copyright, I learned a lot about this subject when I was in T3 training. We were advised to steer away from letting students use ITunes songs due to copyright laws. Personally, I use Googe Images instead of Flickr because Flickr images tend to have more legal restraints.