OS X 10.5 includes a command line audio player (in /usr/bin) called afplay. This is very useful if you want to play a sound file from the command line, shell script, Automator action, etc. The /usr/bin directory is in your path by default, so you can just type afplay file.mp3 to play that file.
afplay makes use of Core Audio, so I think it can play any audio file that QuickTime supports (including mp3, aiff, wav, etc.). If any one is interested, Apple also provides the source code for this application with the Xcode developer tools. You'll find it in this folder: /Developer » Examples » CoreAudio » Services » AudioFileTools.
[robg adds: There's a very simple man page for afplay, which then tells you that help is available with afplay -h. There are a few interesting options, including the ability to play a defined (in time) segment of a file, and to play a file to a defined audio output device.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20081002080543392