WebM Demo - Niagara Falls in 1080p HD:
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."
Te best source of WebM videos is YouTube, first you join the HTML5 experiment. Then you can restrict YouTube search results to WebM videos, by clicking on "Search options" and selecting "WebM".