Are you a big YouTube fan? The YouPlayer extension delivers you all the wild and wacky cats wearing roller skates-like fun of YouTube in your Firefox sidebar.
Here are a few more details from the creator of the extension.
This is a must-have addon for all YouTube fans! YouPlayer is a YouTube video player with playlist similar to the one in Winamp. You may add/drag a YouTube link into a playlist window. When you double-click on it, it will be shown in the video window. You may drag a link to a browser window or right-click it and open in a new tab. You may also add/drag other links, thus the playlist will work as a bookmarker.
I would like to see this be opened up to other video services out there. I think it is a pretty neat idea all around. The only problem is YouTube is the biggest, but not the only one out there as far as Online video goes.