A bookmarklet, Google Chrome extension, and Firefox add-on that allows you to pop out videos from popular sites like:

http://www.youtube.com

http://www.vimeo.com

http://www.megavideo.com

http://video.yahoo.com

etc...

This is the most robust tool of its kind out there and also works for videos embedded in forums/blogs.

Visit the project homepage at http://optimalcycling.com/other-projects/ocp-videopopout/ for more information.