YouTube - HTML5 / H.264
If you use the Google Chrome web browser you can now join the YOUTUBE HTML5 Beta group and view YOUTUBE videos using HTML5's new embedded video features instead of the normal FLASH video wrapper.
This removes an extra layer from the H.264 video playback, theoretically making video playback smoother and more reliable.
I should point out that although I am a Firefox 3.6 (released today) fan, the Google Chrome browser is my preferred browser when playing almost any kind of video.