Install Arek's OggVorbis QuickTime component, and you can listen to Ogg Vorbis files in QuickTime 7. Tested with QuickTime 7.0.2 and iTunes 5.0.1 on OS X 10.4.2.
[robg adds: Back in 2002, this hint covered another Ogg Vorbis plug-in. In checking their project page, though, it was last updated in May of 2004, and only claims compatibility with QuickTime 6.0.2. I have not tested this one...]
December 2005 Addendum: The new URL for this project is:
http://www.xiph.org/quicktime/
The old one above will no longer work...
Mac OS X Hints
http://hints.macworld.com/article.php?story=20051005112822984