Coolplayer

From http://www.daansystems.com/
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.
Features:
GNU General Public License
Freeform Skins
Simple User Interface
Advanced Playlist editor
Internet streaming
ID3 Multitagger
File Renamer
Fast mp3->wav converter
MAD mpeg engine
OGG Vorbis support
Winamp input plugins support
Smallest executable programmed in blazing fast 'C'
Continuous play
8 band precision equalizer
Lot's more...
Try it to know it... Its just 548Kb of a elephant ;)