SoundApp is a handy utility that can play and convert a wide range of sound files including MP3. Requires system 7.5 and Quicktime.

MacAmp is an MPEG Layer 3 (a.k.a. MPEG-3 or MP3) audio player for the Macintosh PowerPC. Requires a PowerPC Mac.

MacPlay is a no-frills MP3 player capable of playing files from a local drive or from a server on the Internet. MacPlay is one of the fastest MP3 players available and is unlikely to skip even on slower machines. Minimum requirment is System 7.5 and Sound Manager 3.1

MP3 Players for Windows

Mp3 Blaster32 is an MP3 player for Windows, featuring a multicolored, customizable interface. The program can play MP3 files, and also convert MP3 files to WAV files and vice versa.

Winamp is a high-quality MPEG audio player that plays all MPEG Layer 2 and Layer 3 audio streams and combines extensive functionality with an intuitive interface.

For many more MP3 players search for MP3 on Download.com.