The WebM blog is now open.Google has released the On2 VP8 video codec as open source (royalty free, BSD-style), while also launching the WebM container format which combines a VP8 video stream with Vorbis audio. Support for WebM has been enabled on YouTube's HTML5 beta, and you can download patches against ffmpeg as well as DirectShow filters for Windows (Gstreamer plugins are labelled as "coming soon").
You can download patches to apply to ffmpeg, and Google has also made DirectShow filters available for Windows users. There's also an introduction to the WebM VP8 codec SDK, and of course you can get your hands on the code.
EDIT: More exaustive informations @ Planet Xiph