MobiTubia is a Flash Lite video player and YouTube application for the S60 platform. It enables mobile users to access flv clips(read as youtube's videos) using several different methods. Additionally, the application allows users to browse and search for specific content on YouTube right from your mobile making things way easier.
It is not a browser plugin, but a very nice solution to play YouTube on S60 devices. The application is not available yet, but once it is I'll right up in front to get it! :)Here's a video showing it in action
Yet another MobiTubia video from Symbian-Freak: