HTML5 provides <audio> and <video> tags for sound and video content. However, every browser seems to support a different combination of codecs and containers for these tags. Open source projects have of necessity only been able to support open formats, but proprietary vendors have been reluctant to throw their weight behind those open formats.
At GoogleIO today, Google, Mozilla, Opera, and 30+ other partners announced WebM, an open source mashup of the Matroska container format, Vorbis audio codec, and newly-open-sourced VP8 video codec. The intention here is to provide a "safe", open-patent-grant format that both open source and proprietary products can integrate. To that end, the WebM code is licensed under a BSD + patent grant license. And, of course, with Google/YouTube supporting this format, there will be a lot of content available.
So how does this touch Fedora? It looks like current Firefox nightles support WebM, and gstreamer support is in the works; hopefully, this will land in time for Fedora 14. For rpmfusion/ffmpeg users, WebM support is in today's upstream ffmpeg release.