Requires SIMBL and GreaseKit. Tested on Safari.

# YouTube in MP4 via QuickTime Plugin!
# http://www.joeyhagedorn.com/2008/04/16/youtube-in-mp4-via-quicktime-plugin

open -a Safari http://www.joeyhagedorn.com/media/downloads/youtubeQT.user.js


# Download YouTube Videos as MP4 Files
# http://googlesystem.blogspot.com/2008/04/download-youtube-videos-as-mp4-files.html

open -a Safari http://members.optusnet.com.au/lbramsay/youtube_mp4.user.js

1. Youtube QT (http://www.joeyhagedorn.com/media/downloads/youtubeQT.user.js)

// ==UserScript==
// @name           Youtube QT
// @namespace      userscripts.org
// @description    Plays the high-resolution YouTube video in an embedded Quicktime player 
// @version        0.1
// @include        http://youtube.com/watch?*
// @include        http://www.youtube.com/watch?*
// @include        http://*.youtube.com/watch?*
// ==/UserScript==
	
var video_id = null;
var video_hash = null;
var video_player = document.getElementById('movie_player');

if (video_player) {var flash_variables=video_player.attributes.getNamedItem('flashvars');
	if (flash_variables) {var flash_values=flash_variables.value;
		if (flash_values) {var video_id_match=flash_values.match(/video_id=([^(\&|$)]*)/);
			if (video_id_match!=null) video_id=video_id_match[1];
				var video_hash_match=flash_values.match(/t=([^(\&|$)]*)/);
				if (video_hash_match!=null) video_hash=video_hash_match[1];
		}
	}
}

if (video_id==null || video_hash==null) {var args=unsafeWindow.swfArgs;
	if (args) {video_id=args['video_id'];
		video_hash=args['t'];
	}
}

if (video_id==null || video_hash==null) return;

var yt_mp4_path = 'http://www.youtube.com/get_video?fmt=18&video_id='+video_id+'&t='+video_hash;
var div_embed=document.getElementById('watch-player-div');

if (div_embed) {div_embed.innerHTML='<embed src=\''+yt_mp4_path+'\' type="video/mp4" width=480 height=400 scale=\'aspect\'></embed>';
};

2. Download YouTube Videos as MP4 (http://members.optusnet.com.au/lbramsay/youtube_mp4.user.js)

// ==UserScript==
// @name           Download YouTube Videos as MP4
// @description    Adds an option to download YouTube videos.
// @namespace      http://googlesystem.blogspot.com
// @include        http://*.youtube.com/watch?*
// @include        http://youtube.com/watch?*
// @version        0.4
// ==/UserScript==

if ( navigator.userAgent.indexOf("AppleWebKit") != -1 ) {
      if (document.getElementById('download-youtube-video')) return;
    
      var video_id = null;
      var video_hash = null;
      var video_player = document.getElementById('movie_player');
      
      if (video_player) {
        var flash_variables = video_player.attributes.getNamedItem('flashvars');  
        if (flash_variables) {
          var flash_values = flash_variables.value;
          if (flash_values) {
            var video_id_match = flash_values.match(/video_id=([^(\&|$)]*)/);
            if (video_id_match!=null) video_id = video_id_match[1];
            var video_hash_match = flash_values.match(/t=([^(\&|$)]*)/);
            if (video_hash_match!=null) video_hash = video_hash_match[1];        
          }
        }
      }
      
      if (video_id==null || video_hash==null) {
        var args = window.swfArgs;
        if (args) {
          video_id = args['video_id'];
          video_hash = args['t'];  
        }
      }
      
      if (video_id==null || video_hash==null) return;
     
       var yt_mp4_path ='http://www.youtube.com/get_video?fmt=18&video_id='+video_id+'&t='+video_hash; 
       var div_embed = document.getElementById('watch-embed-div');
       if (div_embed) {
          div_embed.innerHTML = div_embed.innerHTML + '<br /> <span id=\'download-youtube-video\'> <a href=\''+yt_mp4_path+'\'>Download as MP4</a> '+ ((navigator.userAgent.indexOf('Safari')!=-1)?'(control-click and select <i>Download linked file as</i>)' : ('(right-click and select <i>Save '+ (navigator.appName=='Microsoft Internet Explorer'?'target':'link') +' as</i>)')) + '</span>';
      }

} else {
    (function () {
    
      if (document.getElementById('download-youtube-video')) return;
    
      var video_id = null;
      var video_hash = null;
      var video_player = document.getElementById('movie_player');
      
      if (video_player) {
        var flash_variables = video_player.attributes.getNamedItem('flashvars');  
        if (flash_variables) {
          var flash_values = flash_variables.value;
          if (flash_values) {
            var video_id_match = flash_values.match(/video_id=([^(\&|$)]*)/);
            if (video_id_match!=null) video_id = video_id_match[1];
            var video_hash_match = flash_values.match(/t=([^(\&|$)]*)/);
            if (video_hash_match!=null) video_hash = video_hash_match[1];        
          }
        }
      }
      
      if (video_id==null || video_hash==null) {
        var args = unsafeWindow.swfArgs;
        if (args) {
          video_id = args['video_id'];
          video_hash = args['t'];  
        }
      }
      
      if (video_id==null || video_hash==null) return;
     
       var yt_mp4_path ='http://www.youtube.com/get_video?fmt=18&amp;video_id='+video_id+'&amp;t='+video_hash; 
       var div_embed = document.getElementById('watch-embed-div');
       if (div_embed) {
          div_embed.innerHTML = div_embed.innerHTML + '<br /> <span id=\'download-youtube-video\'> <a href=\''+yt_mp4_path+'\'>Download as MP4</a> '+ ((navigator.userAgent.indexOf('Safari')!=-1)?'(control-click and select <i>Download linked file as</i>)' : ('(right-click and select <i>Save '+ (navigator.appName=='Microsoft Internet Explorer'?'target':'link') +' as</i>)')) + '</span>';
      }
      
    })();
}