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.
Monty from Xiph.org Foundation, the people behind Theora and Vorbis, have announced their support for Google's WebM container format. "The Xiph.Org Foundation is pleased to announce its support of the WebM open media project as a project launch partner. As announced earlier today at the Google I/O Developer Conference, the WebM format combines the VP8 video codec, the Matroska container, and the Vorbis audio codec developed by Xiph into a high-quality, open, unencumbered format for video delivery on the Web. Xiph will continue to contribute to WebM as a whole and collaborate in its further development and deployment." Remember, people, without the hard work from the boys and girls at Xiph, Google would not have been able to do this.
Thoughts from an x264 developer: http://x264dev.multimedia.cx/?p=377