As you have know, the video you download will be FLV format, which you can transfer to your iPod, MP3 Player.
So converter YouTube videos is more important.
I would recommend you use these tools:
YouTube Downloader and Converter
.Download and convert videos from YouTube, RedTube, YouPorn, Yahoo, Veoh, Dailymotion etc. to any video format like AVI, MPEG-4, WMA, MP3.
.
Download YouTube Videos (Another tool download and convert YouTube, RedTube, YouPorn, Yahoo, Myspace videos)
.
YouTube Downloader for Mac (Designed for Mac OS )
.
These tool can not only help you download YouTube but can help you convert YouTube videos to MP4, and Convert YouTube videos to MP3.Also you can insert YouTube videos into PowerPoint and make a great presentation.