Ok, so you know what YouTube.com is... but have you ever heard of kissyoutube.com?
Let's say you want to use a video clip for a presentation you'd like to have a copy to download on your computer. Then this is the site for you!
Once you find the video you'd like to save, add kiss in front of the URL...
turns into
You'll need to have Java installed in order to use the download feature.
http://www.java.com/getjava
I usually recommend downloading the "HQ" version. The file will be in .mp4 format so you'll need a video player like (1) Quicktime or (2) VLC Media Player.
(1) http://www.apple.com/quicktime/download
(2) http://www.videolan.org/vlc/