I have the same problem I have a great 16:10 monitor but half the screen is basically not used in 4:3.

What I do now if the video is long enough and worth it, just download it using VideoDownloader in FF2.0, play it in VLC or any other player that supports widescreen. And Bob's your uncle.

But there is no such support from the providers I believe.

Comment hidden because you are ignoring dag. (show it anyway)

I've never understood wanting to watch a YouTube video at full screen- the compression artefacts and pixelation make it unwatchable for me.

GoogleVideo scales pretty well though.

You are dealing with the monitor and or the video cards abilty to scale or stretch video. All of the video for YouTube and most all the other vid sites present in 4:3. So it's up to your system to do the stretch. So check your video card or monitors setting to see if you can set it to stretch. Most likely it is now set to 1:1 or Native.

Now having said all that, I personally prefer to watch stuff at OAR (Original Aspect Ratio) 4:3 stretched to 16:9 or 16:10 looks..uh...stretchy, short and fat and stuff. I for one will be glad when the vid sites allow for aspect ratio tags, that would be AW-SOME.