There have been several programs promise to grab Youtube videos,convert and import them into iTunes but then pull a fast one and ask you to pay for a license. iTube and PodTube do all the work for free.
iTube, a program by Benjamin Strahs, requires .Net framework and works exclusively on Windows. You simply paste the YouTube videos URL into the application and it does the rest.
PodTube, a Mac OS X program, downloads, encodes, and adds YouTube videos to your iTunes library but requires Safari to fetch the videos. These programs are free and do not contain spyware or adware.