During the early years of the MP3 format the Fraunhofer institute released
several MP3 codecs (coder/decoder) that worked as plug-ins within audio
frameworks, enabling all kinds of audio processing applications (editors,
recorders, transcoders, etc) to export to MP3.
These codecs were based on the FhG MP3 encoders of the time: l3enc, mp3enc, and later fastenc.
In Windows they plugged into the Audio Compression Manager (ACM) framework,
meanwhile in Mac OS they were associated with QuickTime. The ACM codecs
became so popular (mostly obtained from, eh... "alternative" sources) that
severalfrontendprograms were developed.
A great resource to obtain information about differences between various
ACM codec versions is located at Gabriel Bouvigne's website.