It's a cross-platform music player backend C library. It's meant to be generic enough to be the backend of any music player.
I use it as the backend for Groove Basin  which just might hit milestone 1.0.0 around December 20.
There are a bunch of other flaws in MPD's design which I've carefully fixed in the design of libgroove/groovebasin. I know because Groove Basin used to be an MPD client until I got stuck by all of the issues and then finally ripped out MPD and wrote my own backend to use instead.
I have a WIP blog article that goes into more detail about this stuff. This Winamp article might just be the kick in the pants I need to get it finished :)