Mac: Sure most of us use iTunes to purchase music, but playing that music in iTunes is kind of a pain. For a lot less bloat and a more streamlined experience, give Simplier a try — it's a simple music player that uses your existing iTunes music library.
The look of Simplier is very similar to iTunes; imagine if Cupertino made a lightweight music player-only version of the multimedia player/store. There aren't a lot of features (other than much smaller memory use) but you can choose to share what you're listening to in your Twitter timeline.
The app is $2.99 in the Mac App Store, but if iTunes being so deeply integrated into OS X bothers you, this is a way to get a simple music player without having to rearrange your existing library. It can be worth the three bucks to many users just to keep iTunes closed when not buying music.