10th July 2007
MP3 has been the standard compressed audio format for a long time. However it is inferior quality and a proprietary format. There are many other (and better) formats available, but the best and most flexible is OGG. OGG does video to. It is open source and can be included in any open source player. I have been using the OGG vorbis audio codec for some time and can affirm it is significantly better than mp3. It is even IMHO better than aac, commonly used in mp4 files on ipods. If you are buying a portable media device, check if it will play OGG files, or can have software installed the will. You won't regret it.
One player that does play OGG and is cross platform is VLC player .
For more info on the OGG format see the Wikipedia entry.