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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.
epicempire

iTube is an easy to use Windows program that will enable you to transfer any YouTube video onto your iPod video.

Simply enter in the YouTube page URL and iTube will download the file, convert it, and then import it directly into iTunes. The next time you plug in your iPod video it will automatically transfer. If you don't have an iPod video, you can still watch the movie in iTunes.
Download link

PodTube v1.0
What? Encode any YouTube video for your iPod.
And? Simply and quickly download, encode
for iPod and add to your iTunes library any
YouTube video you browse with,
Safari, in a single click!
So, is it good? You let me know...
Downloadlink