MikMod is a module/tracker music player originally written by Jean-Paul Mikkers, and currently maintained by Miodrag Vallat.
It is one of the best known module players in existence if you look at the UNIX side of the world; For long time, MikMod was the best module player there, even when it was shareware (I think the source was still available at least at one point). It came, it slew. Then, at some point, Mikkers didn't maintain the thing anymore and let us have it. And lo, people hacked it and created all sorts of things to make it even frostier. Today, it rules.
MikMod 3.x is free; the player program itself is under GPL, and the library to play modules - libmikmod - is under LGPL.
libmikmod is used in many free software projects, such as XMMS, to play sound and modules. Also, Winamp's module player is based on libmikmod!
MikMod's (older, not the new) logo was created by myself, by the way =) So far, I have only contributed ideas.
Home page: http://www.mikmod.org/