MikMod
A module player and sound library for MOD, S3M, XM and IT that quietly ended up inside countless games.
Started in the mid-1990s by Jean-Paul Mikkers as a DOS module player. The code was later split out into libmikmod, ported to Unix, and became the easiest way for a program to play tracker music without writing its own player routine.
The library handles the usual formats — MOD, S3M, XM and IT — plus a long tail of odd Amiga and PC variants. Through SDL_mixer, libmikmod long provided module support in open-source games, which means a good deal of scene music reached more people as game soundtracks than as releases.