Mac: It's no secret that iTunes is a bloated mess. If you're looking for something simpler, Vox is a music player that taps into your iTunes library, but doesn't come with nearly as much extra junk as iTunes.
Vox has been around for a while, but today it officially leaves beta with version 1.0, so we thought it deserved another mention. Vox is all about keeping things simple: Unlike iTunes, all you get is a small window with your library, and that's essentially it. From there, the compact interface does everything you need it to but doesn't waste a lot of space. In the menus, you get access to everything you'd expect from EQ to Last.fm support, but none of it's in your way when you just want to play music.
Vox also has a lot of preferences, so you can make it play nice with launchers like Quicksilver, or force it to use media buttons on your keyboard. If you're looking for a replacement to iTunes, Vox is worth checking out.