http://www.youtube.com/watch?v=zlfKdbWwruY (standard format - mono, resolution: 320x180)
http://www.youtube.com/watch?v=zlfKdbWwruY&fmt=18 (stereo, resolution: 480x270)
http://www.youtube.com/watch?v=zlfKdbWwruY&fmt=22 (new format - stereo, resolution: 1280x720)
To embed a higher quality version of a video, replace "http://www.youtube.com/v/VIDEO_ID" with "http://www.youtube.com/v/VIDEO_ID&ap=%2526fmt%3D18" or "http://www.youtube.com/v/VIDEO_ID&ap=%2526fmt%3D22" in YouTube's embeddable code.
Unfortunately, the new format (fmt=22) is not available for too many videos, probably because it's still experimental and most people upload compressed videos.
{ via Wired }