While Windows Phone 7 is able to stream YouTube videos, it requires a separate download from the Windows Phone Marketplace anyway to enable this multimedia feature; and the YouTube Mobile video quality and user experience is everything than good because it seems Microsoft's YouTube application is able to stream standard definition (SD) videos only. However, yesterday there was a new YouTube player launched called "SuperTube". SuperTube allows to browse through high definition (HD at 720p at 360p) and standard videos as well as downloading all videos to the phone's local storage, including resuming interrupted downloads.
It's coming with its own stand-alone user interface which allows to search all videos by relevance, publish date, view count and rating and support multi-languages well as YouTube regions.
SuperTube costs US$ 1.29 but a trial version is also offered which allows to browse the HD and SD videos for free. However, the trial version contains ads and allows a maximum download of 3 videos only. The full version removes all advertisements and allows unlimited video downloads.
Cheers ~ Arne