Sure, the iPhone and the Apple TV provide access to YouTube videos, but this access is not the same as downloading a YouTube video to your hard drive. For that, you can use TubeTV 1.0, a straightforward, basic YouTube video downloader.
Just enter the URL of the YouTube video into TubeTV (or use the Search field) and click on the “Download movie” button, and TubeTV saves the video to your Mac.
TubeTV can also convert the video for your iPhone, Apple TV, or iPod, as well as import the video into your iTunes library.
In order to use TubeTV, you need to to install Perian 1.1, a free open source QuickTime component that will let you use QuickTime to watch Flash video files.