Animoto claims to "turn your photos, video clips, and music into stunning video masterpieces to share with everyone. Fast, free, and shockingly easy!" It's true!!
I used Animoto with 3rd-5th grade students for a booktrailer contest. Students chose their favorite works of literature and created a short video enticing peers to checkout the books for a summer read. Using Animoto was easy, and the kids did a great job. It even gave us the chance to talk about copyright/fair use.
The best booktrailers are linked to the district's book catalog so technically, these booktrailers could be seen by anyone in the world that has Internet access. I highly recommend Animoto. Of course, if you publish student work to the Internet, you must have parents/students sign a release.