Mac: We were fans of Vox when it was originally released because it's a very minimal but powerful music player. Now, Vox has hit version 2.0, and it includes a handful of new features that make it easier to use as your full-time music app.
Vox still supports most file formats, lives mostly in your menu bar, and provides a pretty lightweight place to play your music. Now, you can also afford luxuries such as gapless playback, a better search engine that searches in multiple devices and locations (including your NAS), and a new look that makes Vox a little easier to use. If you don't need all the other junk that's included in iTunes that isn't about music, Vox is a great replacement that keeps things simple.
Vox (free) [iTunes App Store]