2Very Preliminary.
First, a Video portal for Irish Schools, This is moderated so it is white listed by a number of content filters used by schools in Ireland.
http://teacherstv.ie/
You can put up your own video, or forward a link to video hosted elsewhere on the web.
Next, and be careful of copyright if you do this, When you surf the Internet, the browser you use downloads web pages and stores them on your computer in a folder called the "Temporary Internet Files" folder.You may want to save a local copy of a video which you have seen on the web. often you will find a copy of this in the Cache directory There is more on this here:
: Internet Cache | eHow.com http://www.ehow.com/internet-cache/#ixzz1KjsZRvKH

If you want to use a video or more specificaly an excerpt from a video on youtube in your class then the following programs may be of interest
you may want to try this pluging for mozilla. https://addons.mozilla.org/en-US/firefox/addon/flashgot/ This is from Nils on the CESI list
Simon Ahern on the same list suggested the site www.clipconverter.cc"Here you just put in the youtube link,
choose the quality level and output format and it even lets you decide what start time and end time you'd like, which is really useful for just extracting the relevant part of a video. You click on the tick boxes next to Start and End and input your own times. I would normally select FLV format which allows me to import the trimmed video clip into prezi. This has been invaluable." Thanks for this Simon!

Ronnan Herron has sugested a firefox plugin called DownloadHelper, but I have not tried this.