There are infinitely many ways to download YouTube videos in FLV, MP4 and other supported formats. Here is a cool greasemonkey script, which enables users to download YouTube videos directly in MP4 format.
Install this greasemonkey script for Firefox,Opera or Chrome Beta2. Now, you can see a “Download as MP4” link below the Embed code in the right navigation panel. Simply, click this link and save the video in high quality MP4 format.
This is the easiest way to download videos from YouTube. :)
To use this greasemonkey script in Firefox, you must have Greasemonkey add-on.
And if you have no idea about Greasemonkey scripts or add-on, here is another cool option for you : Download YouTube As MP4 (Youtubemp) is a Firefox add-on which does the same task. You can get it from here.