The 100 Best YouTube Videos for Teachers is a nice list of YouTube videos that teachers can use in their classroom. The list is sorted by content area and topic and has a short description about each video. Categories include History, Science, How-To's, Technology, Classroom Management and more.
Like anything, teachers should watch these videos before using them.
If you can't access YouTube at school and want to download the videos, you can use RealPlayer in any browser to download them (it's free).