One recent evening, my husband & I browsed YouTube for all of the old folk and rock'n'roll songs we loved and could remember. I was astounded at the footage we found! How the internet can take up alot of time...
My introduction to YouTube was through the "Numa Numa" song - something silly played at school one day, for which I was seeking a particular video. Somehow, from there I found Introducing the Book, which still makes me giggle.
Other than entertainment value, YouTube has both educational and marketing ideas. Even ALA has posted videos, as well as many public libraries across the country - check out those from the McCracken County Library in Paducah, KY (like, The Adventures of SuperLibrarian). Not everything found is appropriate, of course.
My absolute favorite video, recently re-made, updated and sent out to this group a couple of weeks ago, is by another Web hero of mine, Karl Fisch. I watched Did You Know/Shift Happens at the Adventure of the American Mind Conference, and it made a life-altering impression. Versions of this can be found on Teacher Tube (which is gradually adding things beyond boring lectures) and other places.