Looking for a lightweight MP3 player for OS X

If you prefer a dedicated app rather than use Quick Look, check Vox, the lightweight music app for Mac OS X.
It's free but you can donate.

Quicktime player. Quicktime player.

I've been looking for the perfect lightweight OSX player for a long time and still stick with Cog

Change the update feed to Nightly in the preferences and you'll get 0.08 that runs in High Sierra and lower.

UPDATE: An active fork is now available at https://kode54.net/cog (thanks OleGG for the mentioning it)

This probably isn't what you're looking for, but it so happens that the very lightest of lightweight music players is preinstalled on every Mac. No, it's not QuickTime, and it's not QuickLook—you already knew about those, and they're not as lightweight. We can do better.

Open a Terminal and type:

afplay path/to/song.mp3

Your music will begin playing. No, there's no UI—you said you wanted something lightweight, didn't you?

To stop the song, press ^C.

To pause, press ^Z, then resume with the terminal command fg.

If you want playlists, you can do that too:

afplay song1.mp3 && afplay song2.mp3 && afplay song3.mp3

Enjoy listening to music with the satisfaction that none of your precious processing power is being wasted on modern extravagances.