It seems this wasnt open source till sometime now. Great cause this one is just so simple.
No frills music player is what I will call that. Indeed no installation and no nothing else. Just click and start playing. It can support a huge list of fomats and plays good. The source is avalable and you can join to its development too.
GNU General Public License
Simple User Interface
Advanced Playlist editor
Fast mp3->wav converter
MAD mpeg engine
OGG Vorbis support
Winamp input plugins support
Smallest executable programmed in blazing fast 'C'
8 band precision equalizer
Try it to know it... Its just 548Kb of a elephant ;)