Google already has a option for downloading videos, but you tube does not. This is an instructable describing on how to download the videos off of youtube to your computer for editing, storing, whatever.
It opens up endless possibilities for example you can take existing videos like this one
and crappily edit them to be like this one
Step 1: Get the software
You also need grease monkey, great software that allows you to control websites http://greasemonkey.mozdev.org/
Once grease monkey is installed you need the youtube extension http://www.joshkinberg.com/blog/files/youtube_to_me.user.js