Mac OS X only: Free application PicoPlay is a barebones music player for OS X that plugs into your iTunes library and playlists for quick access to your tunes with low resource requirements.
That's pretty much all there is to it. As the developer of PicoPlay points out:
This player was designed with one purpose in mind: don't distract or use resources that you need for more important things.
The player allows you to pick any playlist you've created in iTunes from a drop-down menu, has the standard play, pause and track skipping, and offers shuffle functionality. That's it. It's trim, but if you don't need the full iTunes functionality eating up precious resources, it's a nice little player. PicoPlay is a free download, Mac OS X only.