Short of writing your own scripts to control mplayer, mpd+ncmpc is the most simple and most reliable (most difficult to crash) open source UNIX-friendly music player solution I've ever used. And over the years I've systematically tried every player I could find inside the package/ports repositories and outside of them.
These days I just use scripts and mplayer. But for a ready-made solution, MPD fills the "simple and reliable" niche completely, in my opinion. On those two criteria it is without parallel.
On Windows, foobar2000 was great but the source was never available.