Well if using Firefox than the best add-on as L4ugh already sad is Easy YouTube Downloader.It's so easy,when you instal it,you just have to play any video on youtube,and under the video next to the Like,Add to and Share buttons you have another button witch says Dowload,click on it and it expands and you choose the quality and it begins the download,so easy! If using some other browser i recomand Orbit Downloader,it's a Download manager,instal it,play any video on any site,then just move the mouse on to the playing video frame and a button apears,you click on it,it expands,you choose quality and it begins the download.Hope i helped with this.