Update 12/03/08: Had to update the script because it was creating problems with non-youtube embeds. Please redownload it. Sorry for the mistake.

This script darkens the background of the page when a (YouTube) video is being played.

Should work automatically for YouTube videos. You can also control the dim manually by double-clicking on a video page (on) and clicking the dimmed page (off).

Screenshot:

Please note that this script is experimental. I might work on it some more to improve it, but I can't make any promises.

Get it here.