I learned about SafeShare.TV from Kelly Tenkely's iLearn Technology blog. YouTube has some outstanding videos, but unfortunately, a lot of other "stuff" that is very inappropriate for school. There are comments that are rude, or obscene, related videos of questionable worth... you get the idea.
SafeShare.TV takes away all of the bad stuff, and creates a unique URL that allows you to safely show your video in class.
Click this link to see a great video of Havasu Falls, and notice that all of the other YouTube content is no longer there to distract your students.
[Video] Havasu Falls Grand Canyon Arizona USA
Now take a look at the same video on YouTube