If you're using a linux distribution, Ogg FLAC/Vorbis or Ogg Theora support is already available to you.
Today, Mozilla employee Christopher Blizzard announced, that (most likely) Firefox 3.1 will feature Ogg audio and video support for all platforms.
This is a huge step for the Ogg formats, since neither Windows, nor Mac OS X come with Ogg support pre-installed.