The question I get asked most often is "How can I save videos from YouTube?". Here's how you do it:
http://www.keepvid.com
Go to that site, plug in the URL of the video and download away. The site can also download videos from Vimeo, blip.tv, Dailymotion and a ton of other user-submitted video sites
The site gives you a list of download options. I usually choose the highest quality option (for YouTube, 1080p MP4, 720p MP4, 480p FLV, 360p MP4, in decreasing order).
It's really all that simple. Happy downloading!
Fango