Cog

While the official build of Cog hasn't received any attention since 2011 — and, in fact, the latest binary on the site is from 2007 — there are still forks in active development. The most active fork appears to be Mamburu's, which was updated less than a day ago at the time of writing.

You'll have to compile it yourself, but it's little hassle and worth it. All I had to do was install ffmpeg through homebrew and open up the project in Xcode.

Cog is a superb small player for OS X which simply plays music files from your file system. None of the library rubbish that iTunes enforces. A small but important detail is that Cog is controlled by the media keys on the Mac keyboard too!