Just to share an expirience:
I digg a way to download youTube movies. Some of are web sites where you put the link from YouTube and it gives you file with strange extension and small size. Some are really complicated.
My way was to use moviesherlock (moviesherlock.com or sm like this - please ask me - will rememeber). Really simple utility, that just works, and only 1 button in software.
I downloaded it from here: http://download.cnet.com/MovieSherlock/3000-13633_4-10906771.html?tag=mncol.
I downloaded even HD movies!

But it looks too easy, has no player, but just works. It downloaded ~12 concurrent movies for me and actually slows down my system. Its possible to do search in moviesherlock (http://moviesherlock.com) and search results differs from what you see in YouTube. Thats strange.
Who knows - youTube search are different for same text? I likes cat (Simons cat / dog).
Why doesn't youtube allows to download its movies without external tools?!! It actually sucks!

Thanks in advance!