The latest in Apple's line of "Killer Apps", which includes iMovie2, iDVD and now iTunes.
Somewhat similar in functionality to the popular mp3 cataloging/encoding/playback application Music Match Jukebox, iTunes lets you pop an audio CD into your Mac, rip it, encode it, and catalog it in a easily sortable database. In addition to this, iTunes also includes cool visualization effects, is compatible with many portable mp3 players, and lets you burn audio CDs by simply creating a playlist and hitting the burn button!
Oh yes, and best of all, it's free! Go download your copy today!