Xilisoft Online Video Downloader is a nice video downloader. It works with not only YouTube, but also with Metacafe, Dailymotion, ESPN, NICONICO, Stickham, Vimeo, Blip.tv, Vidlife, BlogTV, BREAK and Revver. At $9.99, this is one of the cheapest commercial downloaders available, and I can say that it works really well.
This downloader is at its core, a web browser. You can browse YouTube and other sites from the application, and when you are ready to download a video, you can simply click on the "Download" button to add it to the download queue. Another way to add videos is to paste a video URL. One good thing about this app is that it lets you choose the version of the video that you want to download. If you are trying to download an HD video from YouTube, for example, all you have to do is select 720p or 1080p from the drop-down menu and then start the download.
In my testing, downloading videos from YouTube worked like a charm, both in standard definition and high definition. The download speed was OK.
In short, this is a great, affordable video download. It only costs $9.99 and it lets you download videos from many websites.