I have used a number of free YouTube file downloaders. Some came with malware, some kept on nagging you for "upgrading", others did not work very well. In the end I gave in and paid for YouTubeByClick. I wish I had done it earlier! It is simple to use, quite fast, and even converts the clip to the format you need. An excellent piece of software! Highly recommended!
The offered Youtube MP3 toolbar is powered by Conduit and the website itself is powered by WorldStream IPv4.8, which is a Dutch internet service that hosts malware and spam.