Saw this yesterday and thought it was cool. The All-In-One Video Bookmarklet lets you download videos from YouTube, Google Video, and a number of other Web sites. You save the bookmarklet (go to the above link, and then on that page right-click on the All-In-One Video Bookmarklet link and pick the bookmarking option), go to the Web page of a YouTube/Google/whatever video you want to save, and then hit the bookmark. You'll get a page which allows you to download the movie in one or more formats.
If you want to try this out, here's a link to a short and funny video about a cat with depth perception problems (remember that you'll have to go save the bookmarklet first). That video makes me laugh out loud every single time (I think it's the sound that really does it for me).