Just as something to play with, try VideoBox from LiveMixing. Besides playing and downloading YouTube, it also lets you play and download video from Vimeo and some Asian video sites. It is trying to be a social networking app geared to sharing video you find with friends, and a bonus is that it also includes a pretty capable video player that plays mp4s from your PDN's SD card. I haven't used the social networking stuff, but it seems to do a pretty good job at playing on-line and local videos.