Internet Explorer 9 will be free later today and one of the many new skin is the inhabitant support for videos. Unfortunately for Google, Microsoft determined to only support H.264 videos by default, so you can't watch WebM videos without installing added software.
To solve this problem, Google urbanized a WebM plugin for IE9. "They said elephants couldn't ride flying dolphins. They said that one of the world's most accepted browsers couldn't play WebM video in HTML5. They were wrong," mentions Google half-jokingly.
The plugin only works on Windows 7 and Windows Vista, the two operating systems supported by IE9. Google suggests to search for WebM videos on YouTube, but I'm not sure if the plugin was really essential since YouTube's HTML5 player also works with H.264 videos.
Last month, Microsoft unconfined a plugin for watching H.264 videos in Google Chrome that will be useful when Google drops support for the accepted codec.