Femto Music Player v1.2
Femto Music Player, the Python version, is tiny, making it a perfect choice for computers like Raspberry Pi, in fact, the Pi was what it was originally written for. On other computers, all you need is Python 2.7.8 (or similar), and Pygame. How to download and install these is written in the README file inside the zip you can download below.
There is now also a JavaScript version of Femto, called JSFemto. It is also very simple, and easy to use on your own webpages. This uses HTML5 audio and a bit of JavaScript.
The players are HTML / Javascript, or Python, and they are multi-platform.