The Netscape plugin
Modplug is an all-purpose module playback library capable of loading (but not necessarily correctly playing) many different tracker music formats. Its name is derived from its original purpose being a web browser plugin, waaaay back in the Netscape 4 era. (Note that this predated Flash, or even ActiveX viruses, and in the day when seemingly every website required installing some stupid plugin in order to work properly... and people actually thought this was a good idea.) Although the browser plugin has thankfully been abandoned, the underlying playback library is still used by Modplug Player and Modplug Tracker, and the code has been released into the public domain for use in other projects.
Schism Tracker uses a highly customized version of the Modplug library, retrofitted with better Impulse Tracker file playback and a number of bug fixes. Many of Schism's fixes have since been back-ported to OpenMPT, fixing one of the long-standing criticisms of it. (The other criticism that its user interface is terrible is still valid.)