I like VDownloader. It's free and it's really easy to use. I've been using it for years. All you have to do is copy the link for the video into the program and click download and the video starts downloading. You can even set it up to have it automatically paste the video URL into the program when you copy it. I have it set up that way. I also add &hd=1 to the end of the URL to download HD videos.
http://www.vdownloader.com/