Like most schools, Youtube is blocked where I teach...the solution, anyvideoconverter.
The way I have used it in the classroom:
1. Find a video, clip or movie on Youtube
2. Copy and paste the URL into AVC and convert to an flv (flash video) file
3. Create a Prezi presentation and embed the video flv file into the prezi
(Click on the Media bubble on prezi & find the flv file in your computer)
When you are done: There is no embed option on the prezi website. To embed simply choose the share option and place the presentation link within an iframes HTML code.
This method circumvents youtube being blocked and allows you to bring any video clip you desire into your classroom.
Here is a short Prezi I created on Japanese Internment during WWII with an embedded video by Fort Minor that I found on youtube.