Windows Phone 7: For the moment, Microsoft's latest smartphone platform doesn't support Flash. For $3, though, you can search and view videos from YouTube, DailyMotion, Funny or Die and other video sites, in HD when available, with the RealTube app.
RealTube isn't a complete YouTube replacement — and if all you want is YouTube compatibility, you can grab a free Microsoft plug-in that sends links to Google's Flash-free m.youtube.com site. RealTube, however, actually converts Flash videos to Silverlight, and gets a lot of praise for its performance. It supports a variety of video portals, and plays in higher-quality HD formats whenever possible.
You can see RealTube in action in this WMExperts demonstration:
RealTube is a $3 purchase for Windows Phone 7 phones only.