mpg123 is a
real time MPEG 1.0/2.0/2.5 audio player for layers 1,2 and 3 (MPEG 1.0 layer 3 aka MP3 most commonly tested).
Among others working with Linux, MacOSX, FreeBSD, SunOS4.1.3, Solaris 2.5, HPUX 9.x, SGI Irix and Cygwin.
It is free software licensed under LGPL (the officially released sources, some files in subversion repository may differ).
As promised, version 1.5.1 is now out, featuring the 4 announced bug fixes (see news archive and tracker). One may note that with this version we seem to have taken all technical steps to make the XMMS2 plugin official (though the code in mpg123 distribution is outdated, there is the fresh stuff from XMMS2 folks).
For older news see the news archive
A recent test (GNU/Linux OS) with 0.60-beta2 showed 3% to 4% CPU usage on decoding high quality VBR MP3 with a pentiumII laptop with 366MHz, while a pentium 100MHz can easily handle two mpg123 instances in realtime (plus the mixing daemon that works on decoded data).
Plays Layer 3 in stereo on an AMD-486-120Mhz or (of course) a faster machine.
Just for info: mpg123 plays an average 128bps stream, with about 66%
in full quality on an AMD 486-133MHz machine.