If you find a great video on YouTube or Google and you want to use it with your students in school, you may have problems with your school's web filter. The video maybe fine, but the entire site is blocked. Zamzar may be the answer to your problem. Zamzar is a free file conversion site. Notice that I said site as in web site. You don't need to download any software. You will need to do the conversion at home, but at least you have the content for your students.
Zamzar will convert more than online movies. It will also convert music, documents and images. Again, it is free and online. ( http://www.zamzar.com/ )
Jim