There are numerous tricks, tools and online sites to download YouTube videos but this one is simplest of them all, just install the free Firefox addon 'Easy YouTube Video Downloader 1.1' and you can download videos in 3GP, MP4 and High-Definition quality (when available) right from the Youtube video page are browsing.
The latest update bring optimized code with new 3GP download option and graphical buttons for single-click direct downloads in format of your choice.