Xilisoft YouTube HD Video Converter is a video converter and downloader. You can use this application to download videos from YouTube.com and then, optionally, convert them to formats other than FLV and MP4 (which are the formats in which the files are downloaded). The application is basically a web browser. You can go to YouTube.com on it and browse the website just like you would on your default browser. When you are ready to download a video, you can simply click on the "download" button to add it to your download queue. At the bottom-right corner of the screen is a drop-down menu from where you can choose the download format. By default, every video that you download using this app will be converted into an MPEG-4 video, but you can change this to any of the supported formats (there are many) or set it to not convert the video at all.
You may notice an "HD" in this app's title. This means that it can download high definition videos from YouTube. To do that, you simply have to select the resolution of the video on the Flash player like you normally would do it. The HD videos are those with a resolution of 720p or 1080p (Full HD).
In my testing, this app performed great. It downloaded HD videos very quickly and I didn't experience any problems with it. The converted files played well on VLC and they looked just as good as the ones on YouTube.com.