vgmstream is a library for playing a wide variety of audio formats used in video games. It currently comes with source for Winamp, xmplay, Audacious, and foobar2000 plugins, and a command line decoder that outputs PCM .wav.
The best place to look now for active development is in kode54's repo. He also maintains the foobar2000 component.
bnnm has the latest Winamp, xmplay, and command line builds.
Halley's Comet Software