Downverter is application that allows to download youtube videos in High Definition quality(1080p,720p) and automatically convert those videos to MP3, AVI, 3GP, MP4, MOV, WMV for your iPod, PSP, Pocket PC or Mobile Phone.
Downverter is free program and support for all major platforms like Ubuntu (64 Bit and 32 Bit), Windows (XP/Vista/7) and Mac OS X (Leopard,Snow Leopard adn Lion). Not so complicated to use this application, you need to do is enter the YouTube video URL into the box, select the desired media format you want to convert your video into, and then select where to save videos downloaded, Download and Convert.
Download and Install Downverter on Ubuntu 11.10/12.04 and Linux Mint 12
wget http://www.downverter.com/Download/DownloadUbubtu32/ sudo dpkg -i downverter-32.deb
More info about Downverter you can found here