YouTube officially announce that they have started putting the option to download YouTube videos. But there is a catch here, the download option will appear on the video only when the author of that video like to put that. Which means, that is totally dependent and all videos on YouTube are not downloadable. 
The other thing is that the author may offer some price tag on his video so that you can buy that video from him.
There are scripts available to let you download the YouTube videos of your choice, but if you want to go for legal and ethical way then go ahead and download some videos from YouTube without doing any sort of crime.
Thai Tran, Product Manager of YouTube writes:
We’re also testing an option that gives video owners the ability to permit downloading of their videos from YouTube. Partners could choose to offer their video downloads for free or for a small fee paid through Google Checkout. Partners can set prices and decide which license they want to attach to the downloaded video files (for more info on the types of licenses, take a look here).
I think lot many other options will open in coming days for video search tools like YouTube so wait and watch the competition.