In my school YouTube is not blocked, but in many districts it is. I believe TeacherTube is the alterative for those. When I am searching for something, I often search TeacherTube 1st. On YouTube the promoted videos on the side may not be appropriate when you have to pull up a clip.
YouTube is blocked in our schools (shocker) but we do have access to TeacherTube AND SchoolTube. We are often able to find many valuable videos on either of those two sites. However, neither are as popular (or even known about) as YouTube.
I tried TeacherTube as an alternative to YouTube because of filtering issues however I found TeacherTube to be completely unreliable, horribly designed, and very difficult to use. As a result, I stopped using it. I was disappointed that such a good idea was executed so poorly.