I own a copy of the Windows version of Quake3, but since I no longer run Windows on any of my hardware, the game has been sitting around collecting dust.
To play the Windows version of Quake3 in OS X, download and install the 1.32 OS X update from id Software's Quake3 Gold page (click on the "updates" link at the bottom of the page). The installer will create a Quake3 folder in your Applications folder. Copy the "pak0.pk3" file from the /Quake3/baseq3/ folder on the Quake3 CD to the "baseq3" folder in the newly created Quake3 folder. Run Quake3 and enter your CD key and you're all set.
[robg adds: Since id distributes a hybrid PC/Mac CD-ROM, I don't believe they'll have an issue with a single user purchasing one license, but if I'm wrong, I'm sure I'll hear about it.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20030710105843152