I did a search for library and for librarian. Some of my hits were good, some not so good. I liked two especially. The Adventures of Super Librarian was cute, obviously a commercial for the library which shows good use of YouTube to promote your library. http://www.youtube.com/watch?v=Bu-TijjVs_g&feature=related
The other that I liked was OVERDUE - the library doo-wap number. It was a group of singing librarians that used the song to humorously bring in the effect of the patriot act on librarianship.
There are lots of ways that you could use YouTube in the library. You could make a video of where to find things in the library, a group project to make a movie about one of the features of the library, skits about new books to get kids to want to read them. There are only limits on the imagination, and, oh yeah, YouTube is blocked at my school.