OGG Vorbis on Mac OS X
Ugh, while I personally went straight from MP3 to Apples proprietary AAC for my personal encoding needs (out of pure laziness, I readily admit) I did secretly believe that Ogg Vorbis (site unreachable from here at time of posting, alternatively look here) is actually the better and(!) cooler codec, last but not least, because it's free.
So when today I stumbled across a soundfile in ogg-format I didn't think much of it and simply dragged it onto my iTunes window - from which it then bounced back.
Turns out, that Quicktime - and thus iTunes - doesn't handle ogg... I mean, geez! It's good, it's free, why would Apple ignore it? While there is a Quicktime plug-in that enables ogg for Mac OS (I'm listening to the abovementioned file right now through iTunes) this is still far from useable. Mainly because it creates a 15 second, 100%-CPU-Load break before playing any ogg-encoded file. In the end I prefer it, though to switching to VLC or MPlayer, who - as first-class open-source citizens - play ogg without a hitch. Apple, please add native ogg support to (at least) Tiger, thanks!

Re: OGG Vorbis on Mac OS X
What's proprietary about AAC? It's an open standard just like MPEG.
Yes, OGG is free both as in beer and speech, but I doubt demand is high enough for making it a priority on Apple's list...