A few weeks ago, you could read about how Opera wants native video playback in browsers. Now you can try it for yourself!
From Opera Labs:
At Opera, we have built an experimental version that has native support for Ogg Theora. For now, it is only available on Windows. NOTE! Do not choose to start Opera in the installation process. After installing, exit the installation process and start the the experimental build manually.
Please note that this experimental version with video is only available for Windows (at least for now).