musikCube
Featuring a solid core set of functions, musikCube is very lightweight and fast, with an interface that will be immediately familiar to anyone who's used an iTunes or Amarok 1.x style player (that is, nearly everyone). Using a SQLite database, musikCube also handles fairly large collections quite well; even if you have a 40k+ song collection musikCube will handle it with aplomb. It's no longer in active development, but as a result musikCube is insulated from the feature-creep that befalls so many players; in this case not being actively developed could be seen as a virtue. In a time when so many developers seem to take an "everything but the kitchen sink, and sometimes even the sink" approach towards players, musikCube is a refreshing throwback that holds its own. While it is well suited for lower resource machines, musikCube may be worth a look even if you're not specifically looking for a lightweight application.
Caveats: -- Since it uses the proprietary BASS audio library for playback, musikCube is not pure open source. -- Version 1.0 contains a DRM bug that will cause a crash if one attempts to play a DRM-protected track. If you like, visit the wiki for documentation and bug reports.
