People who logged into Youtube today have probably noticed that everybody's favorite internet video site has undergone a slight change—everything has gone widescreen! That's right, now you can watch a dog humping a baby in 16:9, rather than the original boring ol' 4:3. Though, if you'd rather stick to boring ol' 4:3, that's not going away either. [Youtube]
They also have h.264 HD support, if you add the &fmt=22 at the end of a Youtube URL that the original have been uploaded in 1280x720 resolution.
Here's one for you to test, go fullscreen with it to see it in good detail and compare it with the other two available modes, the 320x240 FLV normal version, and the 480x270 h.264 version: