If you like to watch YouTube and want to download the movie, then i think youtube-dl will be your answer. It's a python-based script which can be used to download video from YouTube directly to your computer. You only need to feed the URL of the YouTube movie and it will automatically download them into your current directory.
As YouTube.com videos are in Flash Video format, their extension should be flv. In Linux and other unices, video players using a recent version of ffmpeg can play them. That includes MPlayer, VLC, etc. Those two work under Windows and other platforms, but you could also get a specific FLV player of your taste.