Watch YouTube in higher resolutions

YouTube have apparently been testing higher bitrate encodings of it videos. You can see them if you add a &fmt=8 or &fmt=16 to the video url. Historically, all videos have been delivered to the lowest common denominator, apparently sorenson encoded 320x240. By adding &fmt=6 to the URL, the video is served up in 448x336 resolution and &fmt=18 gives you the iPhone-style MP4 stream.

What videos will actually look better in the higher resolution format is completely dependent on the material that was uploaded to YouTube. Videos uploaded that were already heavily compressed or pre-scaled to 320x240 won't look much different.