Forget about using a third-party Web site or software to download 720p video from YouTube – there’s nothing for you to install other than this bookmarklet:

javascript:if(document.location.href.match(/http:\/\/[a-zA-Z\.]*youtube\.com\/watch/)){document.location.href='http://www.youtube.com/get_video?fmt=22&video_id='+swfArgs['video_id']+'&t='+swfArgs['t']}


or just drag this link to your bookmark: Download YouTube HD

the number on red 22 you can change for other format

NEWS:
– Now it checks the best available YouTube quality in the order of fmt=22=>35=>18=>6=>none
– Now it detects downloaded file type and saves it with the right extension:
fmt 22 and 18 = mp4
fmt 13 and 17 = 3gp
all other fmts = flv

here is a complete fmt list for info:
; // fmt=0 -> flv: 320×240 (flv1) / mp3 1.0 22KHz (same as fmt=5)
; // fmt=5 -> flv: 320×240 (flv1) / mp3 1.0 22KHz
; // fmt=6 -> flv: 480×360 (flv1) / mp3 1.0 44KHz (No more…)
; // fmt=13 -> 3gp: 176×144 (mpg4) / ??? 2.0 8KHz
; // fmt=17 -> 3gp: 176×144 (mpg4) / ??? 1.0 22KHz
; // fmt=18 -> mp4: 480×360 (H264) / AAC 2.0 44KHz
; // fmt=22 -> mp4: 1280×720 (H264) / AAC 2.0 44KHz
; // fmt=34 -> flv: 320×240 (flv?) / ??? 2.0 44KHz (default now)
; // fmt=35 -> flv: 640×380 (flv?) / ??? 2.0 44KHz
; // fmt=37 -> mp4:1920×1080 (H264) / AAC 2.0 44KHz

updated to fmt 37