One big complaint of audiophiles on the Mac platform is the limited support for FLAC, the very high quality, open source lossless audio codec. Apple, of course, prefers its own (proprietary) Apple Lossless format. The X Lossless Decoder offers some help for Mac users, however. Unlike many FLAC tools on the Mac it is a Universal Binary and offers conversion from FLAC/Ogg, Apple Lossless, Monkey's Audio, Wavpack, and TTA to WAV, AIFF, PCM, Ogg, AAC, MP3 and FLAC. If you just want to play FLAC/Ogg files in iTunes, the Xiph component will allow this, but it doesn't work perfectly.
XLD is open source and a free download.
[Via The Lossless Audio Blog]