I think YouTube is a great site to find all kinds of interesting things and kids love it. It is blocked in my district and I support that for students because they would waste lots of time on inappropriate or irrelevant stuff. I do with that they would give teachers access though. It has been explained to me as a bandwidth problem when I ask.
I have found TeacherTube to be a great alternative to YouTube, but I am not sure if it is blocked??? My favorite videos are raps and songs that teachers have made to teach content: Mrs. Burk's Perimeter Rap and 9 Time Table Hip Hop for example. I think kids could learn from these and it would be a great project for students to make their own raps.
I also like this student commercial about what you post on line. It is short, but powerful. I really hope this is not blocked at school. I also found that the Common Craft "In Plain English" videos that we have been using are on TeacherTube.
I know that SchoolTube is not blocked but it did not seem to have as good of resources. I like the videos on TED as far as quality and information, but again they are blocked at school :(