Microsoft made availble the Windows Internet Explorer 9 worldwide, which also comes with native support for videos. Unfortunately for Google, Microsoft decided to only support H.264 videos by default, so you can't watch WebM videos without installing additional software.
To solve this problem, Google developed a WebM plugin for IE9. "WebM for IE9 is a free browser plug-in that enables support for playing WebM videos in Microsoft Internet Explorer 9 on Windows 7 and Windows Vista."
"WebM defines the file container structure, video and audio formats. WebM files consist of video streams compressed with the VP8 video codec and audio streams compressed with the Vorbis audio codec. The WebM file structure is based on the Matroska container."
Google suggests to search for WebM videos on YouTube.
Does't it really requried? Since, YouTube's HTML5 player also works with H.264 videos.
Download: WebM for IE9