Just put this script to execute on a YouTube video page, and download it!

ytdownload.js

!function( ytplayer ){
	
var url = ytplayer.config.args["url_encoded_fmt_stream_map"],
	videoFormats = url.split(','),
	videos = [], 
	props = ['quality', 'type'];
	
	setFormats();
	askToUser();

function setFormats(){
	for( var i in videoFormats ){
		videoFormats[i] = unescape( videoFormats[i] );
		
		var video = { number: i };

		for( var p in props ){
			video[ props[p] ] = setProp( videoFormats[i], props[p] );
		}

		video.downloadURL = videoFormats[i].split('&url=')[1].replace(/;.+\&/g, '&');
		if( !(/sig=/g).test(videoFormats[i]) )
			video.downloadURL = video.downloadURL.replace('sig=', 'signature=');
		else
			video.downloadURL += videoFormats[i].match(/sig=[^&]+/g)[0].replace('sig=', '&signature=');

		videos.push( video );
	}	
}

function setProp( videoFormat, prop ){
	// Unfortunely...
	var regexp = new RegExp(prop +'=[^&]+', 'gi'),
		val = videoFormat.match(regexp)[0].replace('&'+ prop +'=', '');
	return val;
}

function askToUser(){
	var pVal = 'Digite o número do formato que você quer', userChoosen;

	for( var i in videos ){
		pVal += '\n'+ 'N#'+ i +'. Tipo ' + videos[i].type + ' \n\t Qualidade ' + videos[i].quality;
	}

	userChoosen = prompt(pVal, '1');

	window.open( videos[userChoosen].downloadURL );
}

}( ytplayer )