I'm not sure whether it's very popular or not but there is the free as in GPL free video player called Miro.
Miro is a project of the Participatory Culture Foundation, a non-profit organization.
This is a very good video software which integrates well with YouTube and Googlevideo. All you need is a very good internet connection for downloading videos.
The reason I post about this is that there are some emacs videos which are worth downloading to understand how certain features are used. Of course you could do the same on the websites of the above mentioned video sites but the player gets them all in one place for later viewing.
Neat piece of software worth having and I have to confess that I use it a lot more than just Emacs videos. :-)
As for the feature set of Miro, you're better off checking the website here rather than me listing the features.
It is is a 1.x release and there are a few bugs in its UI but none of it is a show stopper that prevents you from seeing videos.
Be aware that the binary download is large.