I've been waiting to use the excellent and free (license free / open source) Ogg Vorbis sound encoding scheme more on OS X, but support in iTunes has been lacking, until now.
Check out www.illadvised.com/~jordy. There you will find a QuickTime plugin that will allow iTunes and QuickTime (v6.0.2) to play Ogg Vorbis files (just drop the plugin into the /Library/Quicktime folder), as well as a command line Ogg Vorbis encoder (converts imported CD audio files to Ogg) and a link to the easier to use GUI Ogg encoder "Ogg Drop." This Plugin is working quite well for me under OS X (10.2.1) with iTunes 3 and Quicktime 6.0.2.
The following song information tags in the Ogg files are correctly recognized in iTunes: Song Title, Artist, Album and Genre. iTunes will organize Ogg files tagged with this information side by side with the existing MP3s in your library (assuming you've turned on that option in iTunes). The only downside is that the visualizations do not respond to Ogg files using this plugin version. I'm also not sure yet if you can burn CDs using Ogg encoding using this plugin or not.
Also, to more easily play Ogg files in OS X, take an Ogg sound file and click on it, then hit Command-I to pull up the information window. Expand the "Open With" section and select iTunes as the application to open your Ogg files. Then select the "Change All" button to make sure that iTunes is your default Ogg player and then you're done.
[Editor's note: Another plugin was covered in a previous hint, but (based on a visit to the website) nothing has happened with it since July, and it mentions some incompatabilities between Ogg and QuickTime. I have not tested this newest plug-in.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20021103065300430