“Google, Mozilla and Opera announced a new open video format today called WebM. As part of the WebM project, Google is is freely licensing the VP8 compression technology. This new open video format will use a modified Matroska video container (.webm). WebM format support is available today in Firefox, Chromium, and Opera development builds. All videos that are 720p or larger, uploaded to YouTube after May 19th, will be be encoded in WebM.”
WebM uses the open Ogg Vorbis sound codec.