http://www.vorbis.com/ for more info
Ogg files are smaller than MP3 at the same quality, and there are no patents on it which means that it is possible to have freeware encoders (illegal with MP3 because the people who make encoders for MP3 have to pay royalties)
WinAMP, WinamPAQ, and ALL Linux based media players will play Ogg.
MS Media player is still lagging behind (MS doesn't seem to like open standards, they have the same problem with their web browser and .PNG graphics files)