The WebM project is dedicated to developing a high-quality, open video format for the web that is freely available to everyone.

The WebM launch is supported by Mozilla, Opera, Google and more than forty other publishers, software and hardware vendors.
Link: The WebM Project : The WebM Project : Welcome to the WebM Project

While at home I can access YouTube [filtered at work] I have opted into the HTML5 video player trial. However so far I can't see much difference between the old and new format. However I guess I can play videos without downloading an additional component for my browser.

I am using ChromePlus at home so the part that says:

Google Chrome (h.264 supported now, WebM enabled version coming soon)
applies to me.