When Google announced WebM, an open video format based on the VP8 codec, Opera and Firefox were the first browsers that released nightly builds that supported WebM. Google released a Chromium build that integrated WebM a day later.
Probably the best source of WebM videos is YouTube. The first thing you need to do is to join the HTML5 experiment. Then you can restrict the YouTube search results to WebM videos, by clicking on "Search options" and selecting "WebM".
Other browsers that include support for WebM videos: Google Chrome 6 Dev Channel and the upcoming Firefox 4 beta.
Even if HTML5 and WebM are important for Google, YouTube takes a pragmatic approach. YouTube says that browsers need to improve the native video support and provide features like robust streaming, content protection, fullscreen video, camera and microphone access. "We're very happy to see such active and enthusiastic discussion about evolving web standards - YouTube is dependent on browser enhancement in order for us to improve the video experience for our users. While HTML5's video support enables us to bring most of the content and features of YouTube to computers and other devices that don't support Flash Player, it does not yet meet all of our needs. Today, Adobe Flash provides the best platform for YouTube's video distribution requirements, which is why our primary video player is built with it," explains YouTube.